Add required Keywords entry to the linphone.desktop file

--- a/share/linphone.desktop.in
+++ b/share/linphone.desktop.in
@@ -7,6 +7,7 @@
 Icon=linphone
 Terminal=false
 Categories=Network;Telephony;
+Keywords=messaging;im;sip;voip;
 MimeType=x-scheme-handler/sip-linphone;x-scheme-handler/sip;
 
 
