Make gmock dep initially not required

Add a 'required: false' when building the gmock dependency so it doesn't
have to be present initially, as the code right below will already fetch
it if not present.  Otherwise, the build will fail before it has a
chance to build the subproject if it needs it.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I20c58f3b5b6978fa6deb69cdf799e34bb4054ce4
diff --git a/meson.build b/meson.build
index a0bbecd..daeca9b 100644
--- a/meson.build
+++ b/meson.build
@@ -62,7 +62,7 @@
 systemd_dep = dependency('systemd')
 
 if(get_option('tests').enabled())
-    gmock_dep = dependency('gmock', disabler: true)
+    gmock_dep = dependency('gmock', disabler: true, required: false)
     gtest_dep = dependency('gtest', main: true, disabler: true, required: false)
 
     if not gtest_dep.found() or not gmock_dep.found()