Quickfix for USE=test with -Werror=odr (stubs, so have no real consequences).

timer.h gets indirectly included and can declare these differently, have not
looked closer yet.
--- a/test/test_utils.c
+++ b/test/test_utils.c
@@ -110,6 +110,6 @@
 #ifndef WIN32_TESTS
-void mp_add_timeout(void) {};
-void mp_rel_time_to_timespec(void) {};
-void mp_time_us(void) {};
-void mp_time_us_to_timespec(void) {};
+void mp_add_timeout(int64_t time_us, double timeout_sec) {};
+struct timespec mp_rel_time_to_timespec(double timeout_sec) {return (struct timespec){0};};
+int64_t mp_time_us(void) {return 0;};
+struct timespec mp_time_us_to_timespec(int64_t time_us) {return (struct timespec){0};};
 #endif
--- a/test/test_utils.h
+++ b/test/test_utils.h
@@ -61,6 +61,6 @@
 #ifndef WIN32_TESTS
-void mp_add_timeout(void);
-void mp_rel_time_to_timespec(void);
-void mp_time_us(void);
-void mp_time_us_to_timespec(void);
+void mp_add_timeout(int64_t time_us, double timeout_sec);
+struct timespec mp_rel_time_to_timespec(double timeout_sec);
+int64_t mp_time_us(void);
+struct timespec mp_time_us_to_timespec(int64_t time_us);
 #endif