perf: pass CFLAGS correctly

For this package CFLAGS have to be passed via EXTRA_CFLAGS.
On arm this bug causes build to fail because no -fPIC is present in CFLAGS.

Signed-off-by: Marek Behún <kabel@blackhole.sk>

package/devel/perf/Makefile

@@ -57,7 +57,7 @@ MAKE_FLAGS = \
 	CROSS_COMPILE="$(TARGET_CROSS)" \
 	CC="$(TARGET_CC)" \
 	LD="$(TARGET_CROSS)ld" \
-	CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS)" \
+	EXTRA_CFLAGS="$(TARGET_CFLAGS) $(TARGET_CPPFLAGS)" \
 	LDFLAGS="$(TARGET_LDFLAGS)" \
 	KBUILD_HOSTCFLAGS="$(HOST_CFLAGS)" \
 	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='') \