toolchain/uClibc: correct a slip of the pen

UCLIB_EXTRA_CFLAGS -> UCLIBC_EXTRA_CFLAGS

Signed-off-by: Xinglong Liao <xinglong.liao@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>