nss: Fix build on Centos 7

Centos 7 has glibc 2.18 and nss-native build fails due to implicit
declaration of function putenv during build. This is because of the
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

  putenv(): _XOPEN_SOURCE
      || /* Glibc since 2.19: */ _DEFAULT_SOURCE
      || /* Glibc versions <= 2.19: */ _SVID_SOURCE

and because nss coreconf/Linux.mk only defines

 -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_POSIX_SOURCE

So on such system with glibc 2.18, neither macro makes putenv()
available. Add -D_XOPEN_SOURCE for the Centos 7 and glibc 2.18
native build case.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Armin Kuster <akuster808@gmail.com>
Cc: Armin Kuster <akuster@mvista.com>
Cc: Khem Raj <raj.khem@gmail.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Ross Burton <ross.burton@arm.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-oe/recipes-support/nss/nss_3.51.1.bb

@@ -55,6 +55,8 @@ TUNE_CCARGS_remove = "-mcpu=cortex-a55+crc -mcpu=cortex-a55 -mcpu=cortex-a55+crc
 
 TARGET_CC_ARCH += "${LDFLAGS}"
 
+CFLAGS_append_class-native = " -D_XOPEN_SOURCE "
+
 do_configure_prepend_libc-musl () {
     sed -i -e '/-DHAVE_SYS_CDEFS_H/d' ${S}/nss/lib/dbm/config/config.mk
 }