Allow including pdf manuals in each Osmocom repositories -doc debian
package, by depending on osmo-gsm-manuals-dev.
Related: OS#3899
Example usage: I4c184c62804c0b805a0a2425a5bd0312e94e49ab (osmo-bts.git)
Change-Id: I7edb5093e5b58eb3b0f7af2376476db4026db735