package/gpm: Install libgpm.so to fix gpm detection in package links

package/gpm: Install libgpm.so to fix gpm detection in package links

The unversioned library was missing in the build system, part of the fix
was backported from upstream, the final patch was sent upstream.

Fixes linking against gpm by the links package which fails to detect
gpm support when libgpm.so is not present.

Configure log from links without this patch:
checking for gpm.h... yes
checking for Gpm_Open in -lgpm... no
checking for Gpm_GetLibVersion... no

Configure log from links with this patch:
checking for gpm.h... yes
checking for Gpm_Open in -lgpm... yes
checking for Gpm_GetLibVersion... yes
checking for Gpm_Event wdx and wdy... yes

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>