main.js: Don't import NetworkAgent unless nm_agent support is enabled.

[SparkyBluefang]

When networkmanager is not present, imports.gi.NM will raise an exception and cause cinnamon to die.

(cinnamon:1886): Cjs-CRITICAL **: 23:43:31.458: JS ERROR: Error: Requiring NM, version none: Typelib file for namespace 'NM' (any version) not found
@/usr/share/cinnamon/js/ui/networkAgent.js:7:12
@/usr/share/cinnamon/js/ui/main.js:115:22
@/usr/share/cinnamon/js/misc/util.js:19:14
overrideGObject@/usr/share/cinnamon/js/ui/overrides.js:29:32
init@/usr/share/cinnamon/js/ui/overrides.js:14:5
init@/usr/share/cinnamon/js/ui/environment.js:450:15
@<main>:1:24

** Message: 23:43:31.458: Execution of main.js threw exception: Script <main> threw an exception