Bump required glib build dependency

Bump required glib build dependency

While meson.build indicates that 2.66 is enough, one of the tests
introduced API that is only available to 2.68 which leads to package
build failure:

../../../testsuite/reftests/gtk-reftest.c: In function ‘log_writer’:
../../../testsuite/reftests/gtk-reftest.c:537:7: warning: implicit declaration of function ‘g_log_writer_default_would_drop’; did you mean ‘g_log_writer_default’? [-Wimplicit-function-declaration]
  537 |  if (!g_log_writer_default_would_drop (log_level, NULL))
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |       g_log_writer_default
../../../testsuite/reftests/gtk-reftest.c:537:7: warning: nested extern declaration of ‘g_log_writer_default_would_drop’ [-Wnested-externs]