Build Exiv2 without libinih.

Recently, Exiv2 removed some files from their tree to make it a proper
dependency: Exiv2/exiv2#2443

Though a comment in this MR seems to say it should be a hard dependency,
this other patch says it only disable readExiv2Config() which makes
Exiv2 not read custom user config files: Exiv2/exiv2#2465
(also verified by grepping EXV_ENABLE_INIH which indeed shows it is only
used by this one function)

I assume this is the config file which is discussed of:
https://exiv2.org/manpage.html#config_file

For GIMP, I am unsure if this custom optional config file is that
important and if anyone actually makes use of this. Even more in the
flatpak, where eventually the access to $HOME (where the user file would
be) is meant to disappear for sandboxing reasons. So instead of adding
this additional dep, let's just disable the user config file feature.

org.gimp.GIMP.json

@@ -315,7 +315,8 @@
                     "name": "exiv2",
                     "buildsystem": "cmake-ninja",
                     "config-opts": [
-                        "-DEXIV2_ENABLE_BMFF=ON"
+                        "-DEXIV2_ENABLE_BMFF=ON",
+                        "-DEXIV2_ENABLE_INIH=OFF"
                     ],
                     "builddir": true,
                     "cleanup": [