qga: fitering out -fstack-protector-strong

configure script may add -fstack-protector-strong option instead
of -fstack-protector-all, depending on availability ( see
commit 63678e17 ). Both options have to by filtered out for
qga-vss.dll, otherwise MinGW cross-compilation fails at linking
stage.

Signed-off-by: Joseph Hindin <jhindin@daynix.com>
Message-Id: <1427906337-20805-2-git-send-email-jhindin@daynix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>