Update asterisk.spec
    
    Add the make install-headers target and include the files in the asterisk-devel rpm.

    Upstream commit that breaks out install and install-headers: https://github.com/asterisk/asterisk/commit/c781806e2608f16775e9c0d291fa365605d8ad81?branch=c781806e2608f16775e9c0d291fa365605d8ad81&diff=unified