audio/ardour6: fix build on powerpc

ld: error: can't create dynamic relocation R_PPC_ADDR32 against local symbol in readonly segment; recompile object files with -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output

defined in libs/ardour/cycle_timer.cc.1.o
referenced by cycle_timer.cc

libs/ardour/cycle_timer.cc.1.o:(__ftr_fixup+0x1C)