crypt: Use NSPR header files in addition to NSS header files [BZ #17956]

When configuring and building GNU libc using the Mozilla NSS library
for cryptography (--enable-nss-crypt option), also include the
NSPR header files along with the Mozilla NSS library header files.

Finally, when running the check-local-headers test, ignore the
Mozilla NSPR library header files (used by the Mozilla NSS library).

(cherry picked from commit 57b4af19)