ninja: Fix static library installation dir

Trivially correct typo fix. Didn't actually break anything because the
fallback is libdir anyway, and we always install static libraries to
libdir.

Pointed out by Zhe Wang (0x1997).

mesonbuild/backend/ninjabackend.py

@@ -516,7 +516,7 @@ int dummy;
                              [], False, '']
                         d.targets.append(i)
                     outdir = self.environment.get_shared_lib_dir()
-                elif isinstance(t, build.SharedLibrary):
+                elif isinstance(t, build.StaticLibrary):
                     outdir = self.environment.get_static_lib_dir()
                 elif isinstance(t, build.Executable):
                     outdir = self.environment.get_bindir()