lavu: use address-of operator checking clock_gettime

When targeting a recent enough macOS/iOS version that has clock_gettime
it won't be a weak symbol, in which case clang warns for this check
as it's always true:

  warning: address of function 'clock_gettime' will always
  evaluate to 'true'

This warning is silenced by using the address-of operator to make
the intent explicit.

libavutil/time.c

@@ -57,7 +57,7 @@ int64_t av_gettime_relative(void)
 {
 #if HAVE_CLOCK_GETTIME && defined(CLOCK_MONOTONIC)
 #ifdef __APPLE__
-    if (clock_gettime)
+    if (&clock_gettime)
 #endif
     {
         struct timespec ts;
@@ -72,7 +72,7 @@ int av_gettime_relative_is_monotonic(void)
 {
 #if HAVE_CLOCK_GETTIME && defined(CLOCK_MONOTONIC)
 #ifdef __APPLE__
-    if (!clock_gettime)
+    if (!&clock_gettime)
         return 0;
 #endif
     return 1;