ext-toolchains: fix libnss_*.so installation with external glibc

Commit 7192668 introduced a wrong spelling of BR2_TOOLCHAIN_EXTERNAL_GLIBC
that prevented libnss_files.so and libnss_dns.so from being installed.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/external-toolchain/ext-tool.mk

@@ -281,7 +281,7 @@ check_cross_compiler_exists = \
 uclibc: dependencies $(STAMP_DIR)/ext-toolchain-installed
 
 LIB_EXTERNAL_LIBS=ld*.so libc.so libcrypt.so libdl.so libgcc_s.so libm.so libnsl.so libresolv.so librt.so libutil.so
-ifeq ($(BR2_TOOLCHAIN_EXTERNAL_GCLIBC),y)
+ifeq ($(BR2_TOOLCHAIN_EXTERNAL_GLIBC),y)
 LIB_EXTERNAL_LIBS+=libnss_files.so libnss_dns.so
 endif