Include time.h in conc059_c (#19431)

The test probably could have used `usleep` from `unistd.h` instead, but
this seemed like the simplest solution.

Fixes #19431.

testsuite/tests/concurrent/should_run/conc059_c.c

@@ -7,6 +7,8 @@
 #include <stdio.h>
 #if mingw32_HOST_OS
 #include <windows.h>
+#else
+#include <time.h> // nanosleep
 #endif
 
 void millisleep(int milliseconds);