move sleep_millisec to git-compat-util.h

The sleep function is defined in wrapper.c, so it makes more sense to be a in
system compatibility header.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1960,7 +1960,6 @@ int stat_validity_check(struct stat_validity *sv, const char *path);
 void stat_validity_update(struct stat_validity *sv, int fd);
 
 int versioncmp(const char *s1, const char *s2);
-void sleep_millisec(int millisec);
 
 /*
  * Create a directory and (if share is nonzero) adjust its permissions

git-compat-util.h

@@ -1352,4 +1352,6 @@ static inline void *container_of_or_null_offset(void *ptr, size_t offset)
 	((uintptr_t)&(ptr)->member - (uintptr_t)(ptr))
 #endif /* !__GNUC__ */
 
+void sleep_millisec(int millisec);
+
 #endif