graphics/gstreamer1-plugins-qt: fix build on GCC architectures

C++11 compiler is necessary:
cc1plus: error: unrecognized command line option "-std=c++11"

Description of fields to fill in above: 76 columns --|
PR: If and which Problem Report is related.
Submitted by: If someone else sent in the change.
Reported by: If someone else reported the issue.
Reviewed by: If someone else reviewed your modification.
Approved by: If you needed approval for this commit.
Obtained from: If the change is from a third party.
MFC after: N [day[s]|week[s]|month[s]]. Request a reminder email.
MFH: Ports tree branch name. Request approval for merge.
Relnotes: Set to 'yes' for mention in release notes.
Security: Vulnerability reference (one per line) or description.
Sponsored by: If the change was sponsored by an organization (each collaborator).
Differential Revision: https://reviews.freebsd.org/D### (*full* phabric URL needed).
Empty fields above will be automatically removed.

M gstreamer1-plugins/Makefile.common