Declare capability_name() as returning const char *

Declare capability_name() as returning const char *

Only used as an argument to printf.   Clears gcc warnings:
xrandr.c: In function ‘capability_name’:
xrandr.c:237:2: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
xrandr.c:239:2: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
xrandr.c:241:2: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
xrandr.c:243:2: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
xrandr.c:245:5: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>