subsurface: add dependency on gconf

* fails without
  | Package gconf-2.0 was not found in the pkg-config search path.
  | Perhaps you should add the directory containing `gconf-2.0.pc'
  | to the PKG_CONFIG_PATH environment variable
  | No package 'gconf-2.0' found
  | main.c:7:32: fatal error: gconf/gconf-client.h: No such file or directory
  |  #include <gconf/gconf-client.h>

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>