test: fix build when SHOULD_FORK is false

The code in test/cairo-test-runner.c properly takes into account
platforms that do have fork() support, and uses the SHOULD_FORK define
to know whether fork is available or not.

However, this SHOULD_FORK macro is used to guard the inclusion of
<unistd.h>, which is needed to get the prototype of other functions
(namely readlink and getppid), that are used in portions of this file
not guarded by SHOULD_FORK.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/test/cairo-test-runner.c
===================================================================
--- a/test/cairo-test-runner.c
+++ b/test/cairo-test-runner.c
@@ -36,10 +36,10 @@
 #include <pixman.h> /* for version information */
 
 #define SHOULD_FORK HAVE_FORK && HAVE_WAITPID
-#if SHOULD_FORK
 #if HAVE_UNISTD_H
 #include <unistd.h>
 #endif
+#if SHOULD_FORK
 #if HAVE_SIGNAL_H
 #include <signal.h>
 #endif