gcc-cross: Clean up fixed-includes

gcc-cross: Clean up fixed-includes

We had interesting failures where building gcc-cross-powerpc with 5.0 kernel
headers, then building eudev after moving to 5.2 headers failed.

gcc-cross doesn't rebuild when linux-libc-headers changes due to its
listing in SIGGEN_EXCLUDE_SAFE_RECIPE_DEPS. This shouldn't matter but
fixincludes as adding asm-generic/socket.h to its filtered list which
was then replacing the real header with an older version. This mismatch
lead to build failures.

We trust the Linux kernel headers to be ANSI safe so lets just clear out
any headers and trust the originals to be correct.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>