Commit ef388ff1f3 removed 'CMAKE_INSTALL:=1', and this makes the
development files to be not installed anymore on 'staging_dir'.
Being such, packages that needs to link against libminiupnpc fails
to build, because it cannot find the headers and the library.

Adding an InstallDev fixes this.

Build-tested on: ipq806x (R7800)
Run-tested on: ipq806x (R7800)

Signed-off-by: Daniel Bermond <danielbermond@gmail.com>