meson.build: change operator combining bools from + to and

upstream meson stopped allowing combining boolean with the plus
operator, and now requires using the logical and operator

reference:
mesonbuild/meson@43302d3

Fixes: #20632

meson.build

@@ -35,7 +35,7 @@ conf.set10('BUILD_MODE_DEVELOPER', get_option('mode') == 'developer',
 
 want_ossfuzz = get_option('oss-fuzz')
 want_libfuzzer = get_option('llvm-fuzz')
-if want_ossfuzz + want_libfuzzer > 1
+if want_ossfuzz and want_libfuzzer
         error('only one of oss-fuzz or llvm-fuzz can be specified')
 endif