Fix cli build error

Andrea Scarpinorequested to merge
ilpianista/api:bugfix/cli-json-build into main

While building cli from master, I got:

[  7%] Building C object CMakeFiles/spectre.dir/api/c/spectre-marshal.c.o
/home/andrea/projects/cli/api/c/spectre-marshal.c: In function ‘spectre_marshal_read_json’:
/home/andrea/projects/cli/api/c/spectre-marshal.c:1187:14: error: non-floating-point argument in call to function ‘__builtin_isnan’
 1187 |         if (!isnan(defaultType)) {
      |              ^~~~~
/home/andrea/projects/cli/api/c/spectre-marshal.c:1189:17: error: non-floating-point argument in call to function ‘__builtin_isnan’
 1189 |             if (isnan(resultType)) {
      |                 ^~~~~
make[2]: *** [CMakeFiles/spectre.dir/build.make:202: CMakeFiles/spectre.dir/api/c/spectre-marshal.c.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/spectre.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

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