libm.h: comment j0, j1, y0 and y1

Those functions are currently not used by jq, but having them in
libm.h prevents building with the uClibc C library when it doesn't
have DO_XSI_MATH option enabled.

This issue was spotted by the Buildroot autobuilder system:

  http://autobuild.buildroot.org/results/aaf/aaf3c114e0ca3e265ae32c646ba67f01aaf608bd/build-end.log

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>