po: Fix Makefile rules for in-tree builds without configuration

Adding 'update' to the phony targets fixes this error:

$ LANG=C make -C po update
make: Entering directory `/qemu/po'
  LINK  update
/qemu/po/de_DE.po: file not recognized: File format not recognized
collect2: error: ld returned 1 exit status
make: *** [update] Error 1
make: Leaving directory `/qemu/po'

Some other phony targets (build, install) were also added, and the
existing .PHONY statement was moved to a more prominent position at
the beginning of the Makefile.

The patch also fixes a 2nd bug. The default target should be 'all',
but instead 'modules' (from rules.mak) was the default. Fix this by
adding 'all' as a target before any include statement.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

po/Makefile

@@ -4,6 +4,11 @@
 # Set SRC_PATH for in-tree builds without configuration.
 SRC_PATH=..
 
+# The default target must come before any include statements.
+all:
+
+.PHONY:	all build clean install update
+
 -include ../config-host.mak
 include $(SRC_PATH)/rules.mak
 
@@ -45,5 +50,3 @@ $(PO_PATH)/messages.po: $(SRC_PATH)/ui/gtk.c
 
 $(PO_PATH)/%.po: $(PO_PATH)/messages.po
 	$(call quiet-command, msgmerge -q $@ $< > $@.bak && mv $@.bak $@, "  GEN   $@")
-
-.PHONY: clean all