build: fix build on cygwin

On cygwin, <rpc/rpc.h> lives in a different directory than
/usr/include, so anything that uses it must modify CFLAGS.  This
previously tripped up just 'make check', but now that we build
all test programs unconditionally, it also trips up 'make'.

* tests/Makefile.am (virnetmessagetest_CFLAGS): Find rpc headers.
(cherry picked from commit c8982638)

tests/Makefile.am

@@ -447,7 +447,8 @@ commandhelper_LDADD = $(LDADDS)
 
 virnetmessagetest_SOURCES = \
 	virnetmessagetest.c testutils.h testutils.c
-virnetmessagetest_CFLAGS = -Dabs_builddir="\"$(abs_builddir)\"" $(AM_CFLAGS)
+virnetmessagetest_CFLAGS = -Dabs_builddir="\"$(abs_builddir)\"" \
+		$(XDR_CFLAGS) $(AM_CFLAGS)
 virnetmessagetest_LDADD = ../src/libvirt-net-rpc.la $(LDADDS)
 
 virnetsockettest_SOURCES = \