Get Win32 builds to build

Previously, the location of `GIT_WARN_UNUSED_RESULT` was causing it to
be included _after_ a bunch of other headers (namely `src/vector.h`),
which broke the build.

This change does two things:

* Moves the `GIT_WARN_UNUSED_RESULT` above most of the `$include`s in
  `src/common.h`.
* Stops including `vector.h` from `src/win32/path_w32.c` since the
  header itself does not use it.

src/common.h

@@ -30,6 +30,24 @@
 # define __has_builtin(x) 0
 #endif
 
+/**
+ * Declare that a function's return value must be used.
+ *
+ * Used mostly to guard against potential silent bugs at runtime. This is
+ * recommended to be added to functions that:
+ *
+ * - Allocate / reallocate memory. This prevents memory leaks or errors where
+ *   buffers are expected to have grown to a certain size, but could not be
+ *   resized.
+ * - Acquire locks. When a lock cannot be acquired, that will almost certainly
+ *   cause a data race / undefined behavior.
+ */
+#if defined(__GNUC__)
+# define GIT_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
+#else
+# define GIT_WARN_UNUSED_RESULT
+#endif
+
 #include <assert.h>
 #include <errno.h>
 #include <limits.h>
@@ -136,24 +154,6 @@ GIT_INLINE(int) git_error__check_version(const void *structure, unsigned int exp
 #define GIT_ERROR_CHECK_VERSION(S,V,N) if (git_error__check_version(S,V,N) < 0) return -1
 
 /**
- * Declare that a function's return value must be used.
- *
- * Used mostly to guard against potential silent bugs at runtime. This is
- * recommended to be added to functions that:
- *
- * - Allocate / reallocate memory. This prevents memory leaks or errors where
- *   buffers are expected to have grown to a certain size, but could not be
- *   resized.
- * - Acquire locks. When a lock cannot be acquired, that will almost certainly
- *   cause a data race / undefined behavior.
- */
-#if defined(__GNUC__)
-# define GIT_WARN_UNUSED_RESULT __attribute__((warn_unused_result))
-#else
-# define GIT_WARN_UNUSED_RESULT
-#endif
-
-/**
  * Initialize a structure with a version.
  */
 GIT_INLINE(void) git__init_structure(void *structure, size_t len, unsigned int version)

src/win32/path_w32.h

@@ -8,7 +8,6 @@
 #define INCLUDE_win32_path_w32_h__
 
 #include "common.h"
-#include "vector.h"
 
 /**
  * Create a Win32 path (in UCS-2 format) from a UTF-8 string.  If the given