scripts: headers_install: Add sigcontext.h into the bypass list

sigcontext.h is having the CONFIG_64BIT addition due to the way
we are exporting the headers to userspace and use of the sigcontext.h
by the 32-bit userspace applications. Since we export only the single
set of headers we need to carry this file in the bypass list.

Change-Id: Iaaf2ecd5d46994ca0901d31e40104c800acd5d20
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>