r600: silence the signed overflow warning like radeonsi

r600_gpu_load.c: In function ‘r600_gpu_load_thread’:
../../../../src/util/os_time.h:82:7: warning: assuming signed overflow does not occur when assuming that (X + c) >= X is always true [-Wstrict-overflow]
    if (start <= end)

src/gallium/drivers/r600/Makefile.am

@@ -10,7 +10,7 @@ AM_CFLAGS = \
 	$(GALLIUM_DRIVER_CFLAGS) \
 	$(RADEON_CFLAGS) \
 	$(LIBELF_CFLAGS) \
-	-I$(top_srcdir)/src/amd/common
+	-I$(top_srcdir)/src/amd/common -Wstrict-overflow=0
 
 AM_CXXFLAGS = \
 	$(GALLIUM_DRIVER_CXXFLAGS) \