Make my_mktime() public and rename it to tm_to_time_t()

We will use it from the MinGW port's gettimeofday() substitution.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
diff --git a/git-compat-util.h b/git-compat-util.h
index a9a85be..58cdc08 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -147,6 +147,7 @@
 extern void set_warn_routine(void (*routine)(const char *warn, va_list params));
 
 extern int prefixcmp(const char *str, const char *prefix);
+extern time_t tm_to_time_t(const struct tm *tm);
 
 #ifdef NO_MMAP