firefox: properly fix build without freetype-config

* fails with:
  ../../gfx/thebes/gfxFT2Utils.o: In function `gfxFT2LockedFace::GetMetrics(gfxFont::Metrics*, unsigned int*)':
  /OE/build/oe-core/tmp-eglibc/work/i586-oe-linux/firefox/10.0.11esr-r0/mozilla-esr10/gfx/thebes/gfxFT2Utils.cpp:139: undefined reference to `FT_Get_Sfnt_Table'
  /OE/build/oe-core/tmp-eglibc/work/i586-oe-linux/firefox/10.0.11esr-r0/mozilla-esr10/gfx/thebes/gfxFT2Utils.cpp:234: undefined reference to `FT_Get_Sfnt_Table'
  ../../gfx/thebes/gfxFT2Utils.o: In function `gfxFT2LockedFace::GetFontTable(unsigned int, FallibleTArray<unsigned char>&)':
  /OE/build/oe-core/tmp-eglibc/work/i586-oe-linux/firefox/10.0.11esr-r0/mozilla-esr10/gfx/thebes/gfxFT2Utils.cpp:364: undefined reference to `FT_Load_Sfnt_Table'
  /OE/build/oe-core/tmp-eglibc/work/i586-oe-linux/firefox/10.0.11esr-r0/mozilla-esr10/gfx/thebes/gfxFT2Utils.cpp:372: undefined reference to `FT_Load_Sfnt_Table'
  collect2: error: ld returned 1 exit status
  ../../config/rules.mk:1089: recipe for target 'libxul.so' failed

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>