tests: skip redundant -Wstrict-prototypes setup

We inherit the top-level Rules.mak which sets up this flag for us,
so just rely on that.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

test/Rules.mak

@@ -57,7 +57,6 @@ endif
 endif
 
 XCOMMON_CFLAGS := -I$(top_builddir)test -D_GNU_SOURCE
-CFLAG_-Wstrict-prototypes ?= $(call check_gcc,-Wstrict-prototypes,)
 XWARNINGS      += $(CFLAG_-Wstrict-prototypes)
 CFLAGS         := -nostdinc -I$(top_builddir)$(LOCAL_INSTALL_PATH)/usr/include
 CFLAGS         += $(XCOMMON_CFLAGS) $(KERNEL_INCLUDES) $(CC_INC)