ipkg: add patch to fix musl build

Like Romain Naour did for util-linux, this commit adds a patch for the
ipkg package to remove the use of the __P, which is useless for modern
compilers, and not defined by the musl C library.

Fixes:

  http://autobuild.buildroot.net/results/c27d1d1e6a9bb52fead7b48625bb456f42b2f056/



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