Compatibility: declare strtoimax() under NO_STRTOUMAX

The previous one introduced an implementation of the function, but forgot
to add a declaration.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -345,6 +345,8 @@ extern size_t gitstrlcpy(char *, const char *, size_t);
 #ifdef NO_STRTOUMAX
 #define strtoumax gitstrtoumax
 extern uintmax_t gitstrtoumax(const char *, char **, int);
+#define strtoimax gitstrtoimax
+extern intmax_t gitstrtoimax(const char *, char **, int);
 #endif
 
 #ifdef NO_STRTOK_R