package/gdb: gdb+expat build fix

Commit 23413b51 added --with-expat
configure option and expat dependency when BR2_PACKAGE_EXPAT is
configured.  When cross-compiling, gdb configure fails because the host
system libexpat is referenced.

Configuring gdb with
	--with-libexpat-prefix=$(STAGING_DIR)/usr
fixes the reference and allows gdb to build successfully.

Tested with arm-buildroot-linux-gnueabihf toolchain.

Signed-off-by: Doug Kehn <rdkehn@yahoo.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>