CI: suppress leak checks for libfontconfig

CI: suppress leak checks for libfontconfig

For posterity, the reported leak is:
  =================================================================
  ==14034==ERROR: LeakSanitizer: detected memory leaks

  Direct leak of 256 byte(s) in 1 object(s) allocated from:
      #0 0x7ff0354c190f in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
      #1 0x7ff03523e61f in FcPatternObjectInsertElt /root/fontconfig/src/fcpat.c:525
      #2 0x7ff035240846 in FcPatternObjectAddWithBinding /root/fontconfig/src/fcpat.c:711
      #3 0x7ff035245970 in FcPatternAppend /root/fontconfig/src/fcpat.c:1262
      #4 0x7ff035270c77 in FcParsePattern /root/fontconfig/src/fcxml.c:3110
      #5 0x7ff035271292 in FcEndElement /root/fontconfig/src/fcxml.c:3236
      #6 0x7ff02b099354  (/lib/x86_64-linux-gnu/libxml2.so.2+0x17d354)

  Indirect leak of 32 byte(s) in 1 object(s) allocated from:
      #0 0x7ff0354c12d7 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:77
      #1 0x7ff035239d8b in FcValueListCreate /root/fontconfig/src/fcpat.c:136
      #2 0x7ff035240433 in FcPatternObjectAddWithBinding /root/fontconfig/src/fcpat.c:687
      #3 0x7ff035245970 in FcPatternAppend /root/fontconfig/src/fcpat.c:1262
      #4 0x7ff035270c77 in FcParsePattern /root/fontconfig/src/fcxml.c:3110
      #5 0x7ff035271292 in FcEndElement /root/fontconfig/src/fcxml.c:3236
      #6 0x7ff02b099354  (/lib/x86_64-linux-gnu/libxml2.so.2+0x17d354)

  SUMMARY: AddressSanitizer: 288 byte(s) leaked in 2 allocation(s).

xref: https://gitlab.freedesktop.org/fontconfig/fontconfig/-/issues/295#note_1033797