Fix 1990/stig/Makefile for linux

The paths in the shell command did not have './' to it and this no
longer works. I believe it did work in macOS but I no longer remember
for sure.

1990/stig/Makefile

@@ -127,7 +127,7 @@ all: data ${TARGET}
 	supernova deep_magic magic charon pluto
 
 ${PROG}: ${PROG}.c
-	@echo "source stig.sh ; source stig.c" | ${SH}
+	@echo "source ./stig.sh ; source ./stig.c" | ${SH}
 
 # alternative executable
 #