package/libtirpc: install rpc.h

package/libtirpc: install rpc.h

Install rpc.h to avoid the following build failure with host-nfs-utils
in version 2.5.4 raised since commit
0ce30de72fb303768c5865949b8ce9260d0c43f5:

checking rpc/rpc.h usability... no
checking rpc/rpc.h presence... no
checking for rpc/rpc.h... no
configure: error: Header file rpc/rpc.h not found - maybe try building with --enable-tirpc

This build failure is raised by
http://git.linux-nfs.org/?p=steved/nfs-utils.git;a=commit;h=be7365239abf8556773b063730621967b9971708

Fixes:
 - http://autobuild.buildroot.org/results/a5eaf19f58729c4c00a2131b6a0c378c0fc0effb

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>