Include common.h in hashtable.h

Without this, hashtable.h doesn't know what uint32_t is and the
compiler thinks that it's a function type.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

src/hashtable.h

@@ -4,6 +4,7 @@
 #include "git2/common.h"
 #include "git2/oid.h"
 #include "git2/odb.h"
+#include "common.h"
 
 #define GIT_HASHTABLE_HASHES 3