make gconfig: fix build on Fedora-13

Closes #2095

Avoids error like:
/usr/bin/ld: /dot-dot-dot/buildroot/output/build/buildroot-config/kconfig_load.o: undefined reference to symbol 'dlsym@@GLIBC_2.2.5'

Need to add -ldl to list of GTK2 development libraries.

Signed-off-by: Slava Zanko <slavazanko@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>