Fix recipe commences before 1st target in 1991/dds

Well 'first target' :-) but git commit likes to have no more than 50
columns for the summary.

The problem: if one did for instance

	make CC=clang clobber all

they would get something like:

    Makefile:89: *** recipe commences before first target.  Stop.

because of a missing \ at the end of a line in the middle of setting (or
appending to) CSILENCE.

1991/dds/Makefile

@@ -85,7 +85,7 @@ CSILENCE= -Wno-implicit-function-declaration
 #
 ifeq ($(CC),clang)
 #
-CSILENCE+= -Wno-implicit-int-conversion -Wno-missing-variable-declarations
+CSILENCE+= -Wno-implicit-int-conversion -Wno-missing-variable-declarations \
 	-Wno-pedantic -Wno-poison-system-directorie
 #
 CWARN+= -Weverything