osmo-e1-hardware: Fix missing PKG_CONFIG_PATH directory

otherwise we'll get:

make: Entering directory '/build/doc/manuals'
Package osmo-gsm-manuals was not found in the pkg-config search path.
Perhaps you should add the directory containing `osmo-gsm-manuals.pc'
to the PKG_CONFIG_PATH environment variable
No package 'osmo-gsm-manuals' found

Change-Id: I9f3d6e74be9e0fe31a2ee7aad9869ed7568ff6dd
1 file changed