Skip 'test' folder if build_tests is disabled

Seeing a build error when including gtest.h even when tests are
disabled.  This change skips including the tests subdirectory
when tests are disabled.

Tested:
Verified that the build is successful when tests are disabled.

Change-Id: I2dc2c8f7a898e650e402225c95f809ba265d7031
Signed-off-by: Jason M. Bills <jason.m.bills@linux.intel.com>
diff --git a/meson.build b/meson.build
index b53d9ae..52337df 100644
--- a/meson.build
+++ b/meson.build
@@ -138,4 +138,6 @@
 
 subdir('service_files')
 subdir('src')
-subdir('test')
+if not build_tests.disabled()
+  subdir('test')
+endif