tools: fix undeclared identifier FALSE

I'm getting an error when trying to compile master with small ICU:

../tools/icu/iculslocs.cc:153:13: error: use of undeclared identifier 'FALSE'
  *exists = FALSE;
            ^
1 error generated.

The rest of the file is using false instead of FALSE, this seems to be an error so here's the fix.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines