meson: add mpack as a core dependency

Needed for upcoming fvwm <-> module comm changes.

meson.build

@@ -178,7 +178,7 @@ all_found_deps = []
 
 # Required dependencies, we can easily iterate over these.
 summary_depvals = {}
-all_req_deps = ['fontconfig', 'ice', 'libevent', 'x11', 'xft', 'xrandr', 'xt']
+all_req_deps = ['fontconfig', 'ice', 'libevent', 'mpack', 'x11', 'xft', 'xrandr', 'xt']
 foreach rd : all_req_deps
     this_dep = dependency(rd, required: true)
     summary_depvals += {rd: this_dep}