Meson: try to find udis86 through pkgconfig, fallback to subproject

Only the fork provides a .pc file, so there's no risk of linking the wrong
lib version. If pkg-config can't find it (most cases), fall back to using
the subproject through the wrap file.

subprojects/udis86.wrap

@@ -0,0 +1,5 @@
+[wrap-file]
+method = cmake
+
+[provide]
+udis86 = libudis86_dep