configure: Include time.h when checking for gmtime_r and localtime_r

These functions are available in time.h (conditional on posix thread
safe functions) on mingw.

Previously, these functions weren't detected by configure, and
libavutil/time_internal.h provided replacements, even if time.h
actually contained definitions of them.

These mingw inline functions are currently defined as
 "extern __inline __attribute__((__gnu_inline__))". In this case,
redefining a new static inline version of the same function with the
same name is tolerated.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -4543,9 +4543,7 @@ check_func  gethrtime
 check_func  getopt
 check_func  getrusage
 check_func  gettimeofday
-check_func  gmtime_r
 check_func  isatty
-check_func  localtime_r
 check_func  mkstemp
 check_func  mmap
 check_func  mprotect
@@ -4561,6 +4559,8 @@ check_func  usleep
 check_func_headers io.h setmode
 check_func_headers mach/mach_time.h mach_absolute_time
 check_func_headers stdlib.h getenv
+check_func_headers time.h gmtime_r
+check_func_headers time.h localtime_r
 
 check_func_headers windows.h GetProcessAffinityMask
 check_func_headers windows.h GetProcessTimes