completions/pkg: Only exit for Solaris, not everything-but-FreeBSD

In e8b6705067 this was made to exit if
not on FreeBSD because Solaris has a tool called "pkg" that apparently
"isn't worth supporting".

Since at least DragonflyBSD also uses FreeBSD's pkg thing, let's turn
that check around.

share/completions/pkg.fish

@@ -1,6 +1,8 @@
 # Completions for pkgng package manager
 
-if uname | not string match -q FreeBSD
+# Solaris has a thing called "pkg", it works quite differently,
+# and spews errors when called like this.
+if uname | string match -q SunOS
     exit
 end