selftests/powerpc: Specify we expect to build with std=gnu99

We have some tests that assume we're using std=gnu99, which is fine on
most compilers, but some old compilers use a different default.

So make it explicit that we want to use std=gnu99.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/Makefile

@@ -8,7 +8,7 @@ ifeq ($(ARCH),powerpc)
 
 GIT_VERSION = $(shell git describe --always --long --dirty || echo "unknown")
 
-CFLAGS := -Wall -O2 -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(CURDIR) $(CFLAGS)
+CFLAGS := -std=gnu99 -Wall -O2 -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(CURDIR) $(CFLAGS)
 
 export CFLAGS