Make byteReverse hidden

When building on big-endian systems, the byteReverse symbol becomes an exported symbol of the library. This seems like it's not intended, specially since there are no symbol on little-endian systems (where byteReverse is just an empty define).

src/fcmd5.h already includes src/fcint.h which has a FcPrivate define to hide symbols (when possible) that is already widely used, so use that. (fontconfig/fcprivate.h defines FC_ATTRIBUTE_VISIBILITY_HIDDEN but it seems basically unused.)

On debian package builds (on big-endian archs) this extra unexpected symbol gets caught:

[...]
dh_makeshlibs -plibfontconfig1 -V"libfontconfig1 (>= 2.13)" --add-udeb="fontconfig-udeb" -- -c4
dpkg-gensymbols: error: some new symbols appeared in the symbols file: see diff output below
dpkg-gensymbols: warning: debian/libfontconfig1/DEBIAN/symbols doesn't match completely debian/libfontconfig1.symbols
--- a/debian/libfontconfig1.symbols (libfontconfig1_2.14.1-2_sparc64)
+++ b/debian/libfontconfig1.symbols	2023-01-05 14:08:46.386909057 +0000
@@ -237,5 +237,6 @@
  FcWeightFromOpenTypeDouble@Base 2.13.0
  FcWeightToOpenType@Base 2.12.6
  FcWeightToOpenTypeDouble@Base 2.13.0
+ byteReverse@Base 2.14.1-2
  default_langs@Base 2.12.6
  other_types@Base 2.12.6
dh_makeshlibs: error: failing due to earlier errors
make[1]: *** [debian/rules:16: override_dh_makeshlibs] Error 25
make[1]: Leaving directory '/<<PKGBUILDDIR>>'
make: *** [debian/rules:10: binary-arch] Error 2
dpkg-buildpackage: error: fakeroot debian/rules binary-arch subprocess returned exit status 2