lirc-tools: also install to staging

This commit adds LIRC_TOOLS_INSTALL_STAGING = YES to the lirc-tools
package to get the library headers into sysroot, which is needed to
develop lirc based applications.

Signed-off-by: Michael Schenk <michael.schenk@albistechnologies.com>
[Thomas: rework commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>