compat: remove 'const' from the prototype of timegm().

The prototype of timegm() does not have 'const'.

This fixes compilation on OS/2 which have only a declaration of
timegm().
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

compat/timegm.c

@@ -37,7 +37,7 @@ static bool is_leap_year(unsigned y)
     return true;
 }
 
-time_t timegm(const struct tm *tm)
+time_t timegm(struct tm *tm)
 {
     static const unsigned ydays[12 + 1] = {
         0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334

include/vlc_fixups.h

@@ -200,7 +200,7 @@ struct tm *localtime_r (const time_t *, struct tm *);
 #endif
 
 #ifndef HAVE_TIMEGM
-time_t timegm(const struct tm *);
+time_t timegm(struct tm *);
 #endif
 
 #ifndef HAVE_TIMESPEC_GET