toolchain/helpers.mk: fix check_glibc sysroot typo

Introduced by 68973cca2 (adjust logic to support Linaro 2012.05)

Reported-by: R Zhong <rzhong@hotmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/helpers.mk

@@ -196,7 +196,7 @@ check_glibc_feature = \
 #
 check_glibc = \
 	SYSROOT_DIR="$(strip $1)"; \
-	if test `find $${SYSDROOT_DIR}/lib/ -maxdepth 1 -name 'ld-linux*.so.*' -o -name 'ld.so.*' | wc -l` -eq 0 ; then \
+	if test `find $${SYSROOT_DIR}/lib/ -maxdepth 1 -name 'ld-linux*.so.*' -o -name 'ld.so.*' | wc -l` -eq 0 ; then \
 		echo "Incorrect selection of the C library"; \
 		exit -1; \
 	fi; \