travis: fix x86 libiscsi detection

travis: fix x86 libiscsi detection

When cross compiling to 32 bit systems, pkg-config has to find 32 bit
libraries. This can be done by one of the following
- Setting PKG_CONFIG_PATH/PKG_CONFIG_LIBDIR to an appropriate directory
- Using a 32 bit pkg-config binary

Choose the later for now by installing a 32 bit pkg-config (which will
replace the 64 bit one). This should fix failing CI on the 32 bit x86
build.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>