win32: cast GetProcAddress to void * before casting

GetProcAddress is prototyped to return a `FARPROC`, which is meant to be
a generic function pointer.  It's literally `int (FAR WINAPI * FARPROC)()`
which gcc complains if you attempt to cast to a `void (*)(GIT_SRWLOCK *)`.
Cast to a `void *` before casting to avoid warnings about the arguments.

src/win32/thread.c

@@ -42,15 +42,15 @@ int git_threads_init(void)
 	HMODULE hModule = GetModuleHandleW(L"kernel32");
 
 	if (hModule) {
-		win32_srwlock_initialize = (win32_srwlock_fn)
+		win32_srwlock_initialize = (win32_srwlock_fn)(void *)
 			GetProcAddress(hModule, "InitializeSRWLock");
-		win32_srwlock_acquire_shared = (win32_srwlock_fn)
+		win32_srwlock_acquire_shared = (win32_srwlock_fn)(void *)
 			GetProcAddress(hModule, "AcquireSRWLockShared");
-		win32_srwlock_release_shared = (win32_srwlock_fn)
+		win32_srwlock_release_shared = (win32_srwlock_fn)(void *)
 			GetProcAddress(hModule, "ReleaseSRWLockShared");
-		win32_srwlock_acquire_exclusive = (win32_srwlock_fn)
+		win32_srwlock_acquire_exclusive = (win32_srwlock_fn)(void *)
 			GetProcAddress(hModule, "AcquireSRWLockExclusive");
-		win32_srwlock_release_exclusive = (win32_srwlock_fn)
+		win32_srwlock_release_exclusive = (win32_srwlock_fn)(void *)
 			GetProcAddress(hModule, "ReleaseSRWLockExclusive");
 	}