[libinput] Problem with meson configure

[Marty Jack]

With -Dlibwacom=false, an error occurs.  It looks like it is because the definition is inside "if have_libwacom".
I'd suggest a fix, but I have no experience with meson to know what the best style would be.

meson --prefix=/usr --buildtype=release ../libinput-1.8.0 --libexecdir=/usr/lib \
-Ddebug-gui=false \
-Ddocumentation=false \
-Dlibwacom=false
...
Meson encountered an error in file meson.build, line 76, column 0:
Unknown variable "dep_libwacom".
make: *** [libinput-makefile:16: configure] Error 1