www/firefox: unbreak build with USE_MOZILLA=-soundtouch after r401981

../../dom/media/Unified_cpp_dom_media0.o: In function `mozilla::AudioStream::~AudioStream()':
/obj/dom/media/Unified_cpp_dom_media0.cpp:(.text+0x44f0): undefined reference to `soundtouch::destroySoundTouchObj(soundtouch::SoundTouch*)'
/usr/bin/ld: ../../dom/media/Unified_cpp_dom_media0.o: relocation R_X86_64_PC32 against `_ZN10soundtouch20destroySoundTouchObjEPNS_10SoundTouchE' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Bad value