Boolean #define-s removed from json-c in 2017.

We don't need to #undef TRUE and FALSE before including json.h as this
redefinition has been removed in upstream json-c.
See json-c commit 0992aac61f8b087efd7094e9ac2b84fa9c040fcd.

Note that it still works even if using an older json-c since the
original code was alreadying #undef-ing these 2 values before redefining
them. So that was anyway useless code and this change should work both
with old or new json-c.

Note: the previous code was still working fine when building on Linux,
but not when cross-compiling on Windows. But this is the correct fix
anyway.

(cherry picked from commit f4fd974)

Author: Jehan

mypaint-brush.c

@@ -35,9 +35,6 @@
 #include "rng-double.h"
 
 #ifdef HAVE_JSON_C
-// Allow the C99 define from json.h
-#undef TRUE
-#undef FALSE
 #include <json.h>
 #endif // HAVE_JSON_C