sys.mk: fix duplicate character in character class of LINTFLAGS

The character class contained two times U+0020, which makes one of them
redundant, so remove the character class. PR toolchain/18248 already
contained these two spaces.

Apparently there is no need to support tabs in this place, otherwise
someone would have noticed it in the 17 years since this pattern had
been added in sys.mk 1.83 from 2004-01-27.

No functional change.

share/mk/sys.mk

@@ -1,4 +1,4 @@
-#	$NetBSD: sys.mk,v 1.146 2021/03/23 22:02:44 christos Exp $
+#	$NetBSD: sys.mk,v 1.147 2021/11/28 00:21:12 rillig Exp $
 #	@(#)sys.mk	8.2 (Berkeley) 3/21/94
 #
 # This file contains the basic rules for make(1) and is read first
@@ -139,7 +139,7 @@ YACC.y?=	${YACC} ${YFLAGS}
 	rm -f ${.PREFIX}.o
 .c.ln:
 	${LINT} ${LINTFLAGS} \
-	    ${CPPFLAGS:C/-([IDU])[  ]*/-\1/Wg:M-[IDU]*} \
+	    ${CPPFLAGS:C/-([IDU]) */-\1/Wg:M-[IDU]*} \
 	    -i ${.IMPSRC}
 
 # C++