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).
diff --git a/src/log.c b/src/log.c
index 31fb930..69e495b 100644
--- a/src/log.c
+++ b/src/log.c
@@ -450,7 +450,7 @@
 			}
 			else if (*str == '%')
 				cformat = LF_TEXT;     // convert this character to a litteral (useful for '%')
-			else if (isdigit(*str) || isblank(*str)) {
+			else if (isdigit(*str) || *str == ' ' || *str == '\t') {
 				/* single '%' followed by blank or digit, send them both */
 				cformat = LF_TEXT;
 				pformat = LF_TEXT; /* finally we include the previous char as well */