Fix typo in condition (if linux) for setting the SETCAP/setcap target

As reported via irc by Nelgin, the setcap target was not being passed
to 'make -C src/sbbs3' by default (on Linux) due to a misplaced paren.

install/GNUmakefile

@@ -80,7 +80,7 @@ endif
 MKFLAGS	+=	os=$(os)
 
 ifndef NOCAP
-ifeq (($os),linux)
+ifeq ($(os),linux)
 	SETCAP 	:=	setcap
 endif
 endif