build-plugin: also accept + in message filename detection
... such as e.g. in /usr/include/c++
Otherwise, in a message such as the following, only the part following c++ is extracted as a filename: /usr/include/c++/8/bits/basic_string.h:420:7:
... such as e.g. in /usr/include/c++
Otherwise, in a message such as the following, only the part following c++ is extracted as a filename: /usr/include/c++/8/bits/basic_string.h:420:7: