Fix "make src/fuser" build
By explicitly specifying the dependency between signals.c and signames.h.
BUILT_SOURCES only works with the standard targets, so if somebody tries to build fuser directly he may end up with a build failure. What's worse, he could do "make -j2 src/signames.h src/fuser" and he could end up with a broken build using a only partially generated signames.h.
And that "somebody" was me when packaging killall in psmisc 22.21... so it actually happens.