Mark Boost Timer as a general-purpose library.

Previously, Boost Timer version 2 was only used for afw unit tests. In
order to migrate ip_diffim (which times production code) from Timer
version 1 to version 2, the compiled library must be available in main.

Author: kfindeisen

configs/boost_timer.cfg

@@ -9,6 +9,6 @@ dependencies = {
 config = lsst.sconsUtils.ExternalConfiguration(
     __file__,
     headers=["boost/timer/timer.hpp"],
-    libs={"main": [], "test": ["boost_timer"]},
+    libs=["boost_timer"],
     eupsProduct="boost",
 )