[emmalloc] Fix verbose output

[kleisauke]

toString() is undeclared.

Fixes a regression introduced in commit 04a0cad.

[kleisauke]

Details:

building:ERROR: Closure compiler run failed:

building:ERROR: /tmp/emscripten_temp_50snxhja/vips-es6.jso3.js:757:85: ERROR - [JSC_UNDEFINED_VARIABLE] variable toString is undeclared
  757|     out("aligned_realloc(ptr=" + ptrToString($0) + ", alignment=" + $1 + ", size=" + toString(Number($2)));
                                                                                            ^^^^^^^^

1 error(s), 0 warning(s)

[sbc100]

Are we missing a test for -sMALLOC=emmalloc-verbose?