MinGW: use POSIX signature of waitpid()

Git's source code expects waitpid() to return a signed int status.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -109,7 +109,7 @@ static inline int mingw_unlink(const char *pathname)
 }
 #define unlink mingw_unlink
 
-static inline int waitpid(pid_t pid, unsigned *status, unsigned options)
+static inline int waitpid(pid_t pid, int *status, unsigned options)
 {
 	if (options == 0)
 		return _cwait(status, pid, 0);