wrapper.c: only define gitmkstemps if needed

When the NO_MKSTEMPS build variable is not set, the gitmkstemps
function is dead code.  Use a preprocessor conditional to only include
the definition when needed.

Noticed by sparse.  ("'gitmkstemps' was not declared. Should it be
static?")

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

wrapper.c

@@ -360,10 +360,12 @@ int git_mkstemp_mode(char *pattern, int mode)
 	return git_mkstemps_mode(pattern, 0, mode);
 }
 
+#ifdef NO_MKSTEMPS
 int gitmkstemps(char *pattern, int suffix_len)
 {
 	return git_mkstemps_mode(pattern, suffix_len, 0600);
 }
+#endif
 
 int xmkstemp_mode(char *template, int mode)
 {