kmod_syms.awk: fix removal of the export list from the symbol table

Print some warning when export is requested for non-existing symbol.

Reviewed by:	emaste
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D32878

sys/conf/kmod_syms.awk

@@ -2,6 +2,7 @@
 
 # Read global symbols from object file.
 BEGIN {
+	modname = ARGV[1]
         while ("${NM:='nm'} -g " ARGV[1] | getline) {
                 if (match($0, /^[^[:space:]]+ [^AU] (.*)$/)) {
                         syms[$3] = $2
@@ -12,7 +13,12 @@ BEGIN {
 
 # De-list symbols from the export list.
 {
-        delete syms[$0]
+	smbl = $0
+	if (!(smbl in syms)) {
+		printf "Symbol %s is not present in %s\n",	\
+		    smbl, modname > "/dev/stderr"
+	}
+	delete syms[smbl]
 }
 
 # Strip commons, make everything else local.