BUILD: log: silent a warning about isblank() with latest patches

Recent commit 06d97f9 (MEDIUM: log-format: relax parsing of '%' followed
by unsupported characters) caused the following warning on some compilers
since isblank is not always present :

  src/log.c: In function 'parse_logformat_string':
  src/log.c:453: warning: implicit declaration of function 'isblank'

As usual, replace it with the two values (space and tab).
1 file changed