test/silly: Extend include path.

When attempting to build uClibc under buildroot, including building the
tests, the silly tests don't currently compile, a result of attempting
to build using a compiler that does not yet have an installed version of
uClibc available.  The error is a missing header file, specifically
atomic.h.

Taking inspiration from the nptl tests, I have extended the EXTRA_CFLAGS
variable to add the required include paths.  The tests can now be built
under buildroot.

Signed-off-by: Andrew Burgess <andrew.burgess@embecosm.com>
Acked-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

test/silly/Makefile.in

@@ -4,5 +4,9 @@
 RET_hello := 42
 RET_tiny  := 42
 
-# missing internal headers, disable these
-TESTS_DISABLED := tst-atomic tst-atomic-long
+atomic_headers := -I$(top_srcdir)libc/sysdeps/linux/$(TARGET_ARCH) \
+	-I$(top_srcdir)libc/sysdeps/linux \
+	-I$(top_builddir)include
+
+CFLAGS_tst-atomic = $(atomic_headers)
+CFLAGS_tst-atomic-long = $(atomic_headers)