installer: Fix backtrace display.

The 'syslog' macro cannot be resolved in the pre-unwind-handler
context. Failed to resolve 'syslog' in this context resulted in the swallowing
of all installation errors.

Fixes 5c04b00cf4.

* gnu/installer.scm (installer-program): Use (gnu installer utils) module.

gnu/installer.scm

@@ -358,6 +358,7 @@ selected keymap."
                          (gnu installer services)
                          (gnu installer timezone)
                          (gnu installer user)
+                         (gnu installer utils)
                          (gnu installer newt)
                          ((gnu installer newt keymap)
                           #:select (keyboard-layout->configuration))