Declare gexiv2 dependency on many targets (due to gimpmetadata.h)

Our build files were relying 'sysroot' to find gexiv2.h but this is
not possible with Apple Clang om which sysroot points to macOS SDK.
So, exotic environments like Homebrew were failing. Let's fix this.

tools/meson.build

@@ -10,7 +10,7 @@ gimptool = executable('gimptool' + exec_ver,
   'gimptool.c',
   include_directories: rootInclude,
   dependencies: [
-    gegl, gtk3,
+    gegl, gtk3, gexiv2
   ],
   link_with: [
     libgimpbase,
@@ -25,7 +25,7 @@ gimp_test_clipboard = executable('gimp-test-clipboard' + exec_ver,
   'gimp-test-clipboard.c',
   include_directories: rootInclude,
   dependencies: [
-    gegl, gtk3,
+    gegl, gtk3, gexiv2
   ],
   install: true,
 )