Revert "Hexagon: fix signal.c compile error"

This reverts commit f3f601c1.

UAPI headers cannot use "uapi/" in their paths by design -- when they're
installed, they do not have the uapi/ prefix.  Otherwise doing so breaks
userland badly.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>