multimedia/libopenshot: fix build on non-OpenMP targets
gcc-c++17-lib is not a valid argument to USES=compiler. Use USE_GCC=yes
instead.
Approved by: portmgr (build fix blanket)
MFH: 2026Q2
(cherry picked from commit 5a70190401dfb4aa5bf0d5752c1e4079f1c45865)