Revert "Do not include non-standardized xlocale.h, include locale.h instead"

This reverts commit 274b2aee, which breaks in
external/icu on macOS, <https://ci.libreoffice.org/job/lo_tb_master_mac/20605/>:

> digitlst.cpp:497:8: error: unknown type name 'locale_t'
> static locale_t gCLocale = (locale_t)0;
>        ^

external/icu/UnpackedTarball_icu.mk

@@ -35,7 +35,6 @@ $(eval $(call gb_UnpackedTarball_add_patches,icu,\
 	external/icu/icu4c-59-werror-undef.patch.1 \
 	external/icu/icu4c-59-werror-shadow.patch.1 \
 	external/icu/icu4c-iOS.patch.1 \
-	external/icu/icu4c-59-icu13329-xlocale.patch.1 \
 ))
 
 $(eval $(call gb_UnpackedTarball_add_file,icu,source/data/brkitr/khmerdict.dict,external/icu/khmerdict.dict))

external/icu/icu4c-59-icu13329-xlocale.patch.1

-# xlocale.h is gone with glibc 2.26 and should never had been included.
-# https://ssl.icu-project.org/trac/ticket/13329
-diff -ur icu.org/source/i18n/digitlst.cpp icu/source/i18n/digitlst.cpp
---- icu.org/source/i18n/digitlst.cpp	2017-01-20 01:20:31.000000000 +0100
-+++ icu/source/i18n/digitlst.cpp	2017-09-12 22:03:15.019034749 +0200
-@@ -64,7 +64,7 @@
- # if U_PLATFORM_USES_ONLY_WIN32_API || U_PLATFORM == U_PF_CYGWIN
- #   include <locale.h>
- # else
--#   include <xlocale.h>
-+#   include <locale.h>
- # endif
- #endif
-