src/libpcp_fault/src/GNUmakefile: fix rules for external files
src/libpcp_fault/src/GNUmakefile: fix rules for external files

$(EXT_FILES) were getting mixed up with $(CFILES) and $(HFILES)
and leading to this sort of prattle:

GNUmakefile:168: warning: overriding recipe for target 'jsmn.h'
GNUmakefile:164: warning: ignoring old recipe for target 'jsmn.h'
GNUmakefile:168: warning: overriding recipe for target 'jsmn.c'
GNUmakefile:164: warning: ignoring old recipe for target 'jsmn.c'
GNUmakefile:168: warning: overriding recipe for target 'sort_r.h'
GNUmakefile:164: warning: ignoring old recipe for target 'sort_r.h'

Use $(filter-out, ...) gmake function to fix this.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp