Use @libdir@ in both of regress/{input,output}/security_label.source

Though @libdir@ almost always matches @abs_builddir@ in this context,
the test could only fail if they differed.  Back-patch to 9.1, where the
test was introduced.

Hamid Quddus Akhtar

src/test/regress/output/security_label.source

@@ -38,7 +38,7 @@ ERROR:  no security label providers have been loaded
 SECURITY LABEL ON ROLE seclabel_user3 IS 'unclassified';			-- fail
 ERROR:  no security label providers have been loaded
 -- Load dummy external security provider
-LOAD '@abs_builddir@/dummy_seclabel@DLSUFFIX@';
+LOAD '@libdir@/dummy_seclabel@DLSUFFIX@';
 --
 -- Test of SECURITY LABEL statement with a plugin
 --