[gentoo] Unmask package

!!! All ebuilds that could satisfy "sys-apps/uam" have been masked.
!!! One of the following masked packages is required to complete your request:
- sys-apps/uam-0.0.3 (masked by: missing keyword)

-arch keyword or -* keyword means that the application does not work on your architecture. If you believe the package does work file a bug at our bugzilla website. 

package.keywords
                     Per-package KEYWORDS.  Useful for mixing unstable packages in with a normally stable system  or  vice  versa.   This  will
                     allow ACCEPT_KEYWORDS to be augmented for a single package.

                     Format:
                     - comment lines begin with # (no inline comments)
                     - one DEPEND atom per line followed by additional KEYWORDS
                     - lines without any KEYWORDS imply unstable host arch

                     Example:
                     # always use unstable libgd
                     media-libs/libgd ~x86
                     # only use stable mplayer
                     media-video/mplayer -~x86
                     # always use unstable netcat
                     net-analyzer/netcat

                     Note:
                     In addition to the normal values from ACCEPT_KEYWORDS package.keywords supports three special tokens:

                     *  package is visible if it is stable on any architecture
                     ~* package is visible if it is in testing on any architecture
                     ** package is always visible (KEYWORDS are ignored completely)

                     Additional  Note:  If  you  encounter the -* KEYWORD, this indicates that the package is known to be broken on all systems
                     which are not otherwise listed in KEYWORDS.  For example, a binary only package which is built for x86 will look like:

                     games-fps/quake3-demo-1.11.ebuild:KEYWORDS="-* x86"

                     If you wish to accept this package anyways, then use one of the other keywords in your package.keywords like this:

                     games-fps/quake3-demo x86