Ensure that OS and ARCH are defined in tests/Makefile.am

For strace v4.10 this is a purely cosmetic fix, since the include
paths derived from $(OS)/${ARCH) are never used. These variables are
correctly defined by default in strace v4.11.

Partial backport of upstream commit:

  http://sourceforge.net/p/strace/code/ci/25c804ce42261b24b19d35c637bf2745c237ee07

Upstream-Status: Backport

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>

Index: strace-4.10/tests/Makefile.am
===================================================================
--- strace-4.10.orig/tests/Makefile.am
+++ strace-4.10/tests/Makefile.am
@@ -1,5 +1,7 @@
 # Automake input for strace tests.
 
+OS = linux
+ARCH = @arch@
 AM_CFLAGS = $(WARN_CFLAGS)
 AM_CPPFLAGS = -I$(top_builddir)/$(OS)/$(ARCH) \
 	      -I$(top_srcdir)/$(OS)/$(ARCH) \