multimedia/gpac: fix build on powerpc64le

ld: error: relocation R_PPC64_TOC16_LO cannot be used against symbol 'gf_sg_handle_dom_event'; recompile with -fPIC

defined in scenegraph/dom_events.o
referenced by dom_events.c

scenegraph/dom_events.o:(gf_dom_listener_build_ex)

ld: error: relocation R_PPC64_TOC16_LO cannot be used against symbol 'gf_sc_on_event'; recompile with -fPIC

defined in compositor/compositor.o
referenced by compositor.c

compositor/compositor.o:(gf_sc_load)

ld: error: relocation R_PPC64_TOC16_LO cannot be used against symbol 'gf_scene_get_time'; recompile with -fPIC

defined in compositor/scene.o
referenced by scene.c

compositor/scene.o:(gf_scene_new)

cc: error: linker command failed with exit code 1 (use -v to see invocation)