contrib/scalar: fix 'all' target in Makefile

Add extra ':' to second 'all' target definition to allow 'scalar' to build.
Without this fix, the 'all:' and 'all::' targets together cause a build
failure when 'scalar' build is enabled with 'INCLUDE_SCALAR':

    Makefile:14: *** target file `all' has both : and :: entries.  Stop.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: vdye

contrib/scalar/Makefile

@@ -11,7 +11,7 @@ include ../../config.mak.uname
 TARGETS = scalar$(X) scalar.o
 GITLIBS = ../../common-main.o ../../libgit.a ../../xdiff/lib.a
 
-all: scalar$(X) ../../bin-wrappers/scalar
+all:: scalar$(X) ../../bin-wrappers/scalar
 
 $(GITLIBS):
 	$(QUIET_SUBDIR0)../.. $(QUIET_SUBDIR1) $(subst ../../,,$@)