buildroot

json-c: add patch to make sure the library is linked against libm when needed

On uClibc, a number of symbols such as isnan() are in libm and not
libc, so we might need to link against libm. This commit adds a patch
to json-c that does this.

Fixes:

  http://autobuild.buildroot.net/results/94d/94dd7f38ae105e7cd2f342d718e27a65edcf3a3d/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>