meson: fix invalid keyword argument warnings

cc.compiles() doesn't have a 'prefix' argument (yet) and the
prefix has already been prepended to the source code snippets.

https://github.com/mesonbuild/meson/issues/2364
diff --git a/meson.build b/meson.build
index e6e315b..684e105 100644
--- a/meson.build
+++ b/meson.build
@@ -220,7 +220,7 @@
 #error Either _POSIX_TIMERS or CLOCK_REALTIME not defined
 #endif
 '''
-if cc.compiles(posix_timers_src, prefix : time_prefix, name : 'posix timers from time.h')
+if cc.compiles(posix_timers_src, name : 'posix timers from time.h')
   cdata.set('HAVE_POSIX_TIMERS', 1)
 endif
 
@@ -229,7 +229,7 @@
 #error Either _POSIX_MONOTONIC_CLOCK or CLOCK_MONOTONIC not defined
 #endif
 '''
-if cc.compiles(monotonic_clock_src, prefix : time_prefix, name : 'monotonic clock from time.h')
+if cc.compiles(monotonic_clock_src, name : 'monotonic clock from time.h')
   cdata.set('HAVE_MONOTONIC_CLOCK', 1)
 endif