Link against libiconv on IRIX

On IRIX, "-liconv" must be added to the linker command line in order to
get iconv(3) support; set the according Makefile variable appropriately.

Signed-off-by: Holger Weiß <holger@zedat.fu-berlin.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -886,6 +886,7 @@ ifeq ($(uname_S),IRIX)
 	SNPRINTF_RETURNS_BOGUS = YesPlease
 	SHELL_PATH = /usr/gnu/bin/bash
 	NEEDS_LIBGEN = YesPlease
+	NEEDS_LIBICONV = YesPlease
 endif
 ifeq ($(uname_S),IRIX64)
 	NO_SETENV=YesPlease
@@ -904,6 +905,7 @@ ifeq ($(uname_S),IRIX64)
 	SNPRINTF_RETURNS_BOGUS = YesPlease
 	SHELL_PATH=/usr/gnu/bin/bash
 	NEEDS_LIBGEN = YesPlease
+	NEEDS_LIBICONV = YesPlease
 endif
 ifeq ($(uname_S),HP-UX)
 	NO_IPV6=YesPlease