lib/libzstd.mk: fix typo in the definition of LIB_BINDIR

lib/libzstd.mk: fix typo in the definition of LIB_BINDIR

Commit f4dbfce79cb2b82fb496fcd2518ecd3315051b7d ("define LIB_SRCDIR
and LIB_BINDIR") significantly reworked the build logic, but in its
introduction of LIB_BINDIR a typo was made.

It was introduced as such:

+LIB_SRCDIR ?= $(dir $(realpath $(lastword $(MAKEFILE_LIST))))
+LIB_BINDIR ?= $(LIBSRC_DIR)

But the definition of LIB_BINDIR has a typo: it should use
$(LIB_SRCDIR) not $(LIBSRC_DIR).

Due to this, $(LIB_BINDIR) is empty, therefore in programs/Makefile,
-L$(LIB_BINDIR) is expanded to just -L, and consequently when trying
to link the "zstd" binary with the libzstd library, it cannot find it:

host/lib/gcc/powerpc64-buildroot-linux-gnu/13.3.0/../../../../powerpc64-buildroot-linux-gnu/bin/ld: cannot find -lzstd: No such file or directory

This commit fixes the build by fixing this typo.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>