Fix gcc-10 warnings for multiple definition of pcmk_handler

/usr/bin/ld: boothd-ticket.o:./src/pacemaker.h:35: multiple definition of `pcmk_handler'; boothd-main.o:./src/pacemaker.h:35: first defined here
/usr/bin/ld: boothd-pacemaker.o:./src/pacemaker.h:35: multiple definition of `pcmk_handler'; boothd-main.o:./src/pacemaker.h:35: first defined here
/usr/bin/ld: boothd-handler.o:./src/pacemaker.h:35: multiple definition of `pcmk_handler'; boothd-main.o:./src/pacemaker.h:35: first defined here
/usr/bin/ld: boothd-attr.o:./src/pacemaker.h:35: multiple definition of `pcmk_handler'; boothd-main.o:./src/pacemaker.h:35: first defined here