quota: fix build with libtirpc

Fixes:
  http://autobuild.buildroot.net/results/bba8faf210414d956cc07629b8ba99ceed0ebdf6/
  http://autobuild.buildroot.net/results/f525b48e2982adc4987cb772be92eeda30569c13/
  http://autobuild.buildroot.net/results/0d2c2cd0bda1dd3c1e137117df535d8e02d67e8b/
  http://autobuild.buildroot.net/results/a815a4f943e6a07f87f8f6bbf9910459575db311/
  http://autobuild.buildroot.net/results/1b72769e65444322de4bfcb02200b7d999f9f5c1/
  http://autobuild.buildroot.net/results/195f6c34904b8185d44611c3d56084cb7d3e8a14/
  http://autobuild.buildroot.net/results/aac02dd6c5ea04ea70cfa6921a04b5558a9bbe7f/
  http://autobuild.buildroot.net/results/fbaec431108bec088ff543771c52f2bc20e15b31/


  [...]

QUOTA_CFLAGS / QUOTA_LIBS shouldn't be quoted since QUOTA_MAKE_OPTS and
QUOTA_CONF_ENV already add quotes.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>