travis.yml: Set G_MESSAGES_DEBUG do report GLib errors

Since commit f5852efa we can display GLib errors with the QEMU
error reporting API. Set it to the 'error' level, as this helps
understanding failures from QEMU calls to GLib on Travis-CI.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200316101544.22361-1-philmd@redhat.com>
Message-Id: <20200316172155.971-6-alex.bennee@linaro.org>

.travis.yml

@@ -79,6 +79,7 @@ env:
     - MAIN_SOFTMMU_TARGETS="aarch64-softmmu,mips64-softmmu,ppc64-softmmu,riscv64-softmmu,s390x-softmmu,x86_64-softmmu"
     - CCACHE_SLOPPINESS="include_file_ctime,include_file_mtime"
     - CCACHE_MAXSIZE=1G
+    - G_MESSAGES_DEBUG=error
 
 
 git: