openjfx11, openjfx15: add -fcommon workaround
Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:
ld: gsttypefindelement.o:(.bss._gst_disable_registry_cache+0x0): multiple definition of
`_gst_disable_registry_cache'; gst.o:(.bss._gst_disable_registry_cache+0x0): first defined here
openjfx17 is not affected.