multimedia/gpac-libgpac: fix build with clang 18

Clang 18 no longer accepts -export-dynamic, which is a linker option,
resulting in an error building multimedia/gpac-libgpac:

cc: error: unknown argument: '-export-dynamic'

Fix this by using the correct spelling, -Wl,--export-dynamic.

PR: 276143
MFH: 2024Q1
(cherry picked from commit a235c289356ba567b7c44591216564c73505cf03)