build: Add missing square brackets on character class in sed expression

Although this is not currently an issue as none of the perl scripts have
spaces after the hashbang, it's still an incorrect expression.

Signed-off-by: Guillem Jover <guillem@debian.org>

scripts/Makefile.am

@@ -109,7 +109,7 @@ nobase_dist_perllib_DATA = \
 man3_MANS =
 
 do_perl_subst = $(AM_V_GEN) \
-		sed -e "s:^\#![:space:]*/usr/bin/perl:\#!$(PERL):" \
+		sed -e "s:^\#![[:space:]]*/usr/bin/perl:\#!$(PERL):" \
 		    -e "s:\$$dpkglibdir[[:space:]]*=[[:space:]]*['\"][^'\"]*['\"]:\$$dpkglibdir=\"$(pkglibdir)\":" \
 		    -e "s:\$$pkgdatadir[[:space:]]*=[[:space:]]*['\"][^'\"]*['\"]:\$$pkgdatadir=\"$(pkgdatadir)\":" \
 		    -e "s:\$$admindir[[:space:]]*=[[:space:]]*['\"][^'\"]*['\"]:\$$admindir=\"$(admindir)\":" \