package/pkg-meson.mk: simplify sed-expression to add to [binaries]

There is no need to memorise the matched pattern; it's a left-over
from a previous attempt.

However, ensure the pattern is correctly anchored to the beginning and
end of the line, to avoid accidentally matching it anywhere else (e,g,
in a comment).

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Peter Seiderer <ps.report@gmx.net>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/pkg-meson.mk

@@ -80,7 +80,7 @@ define $(2)_CONFIGURE_CMDS
 	    -e "s%@TARGET_CXXFLAGS@%$$($(2)_MESON_SED_CXXFLAGS)%g" \
 	    -e "s%@HOST_DIR@%$$(HOST_DIR)%g" \
 	    $$(foreach x,$$($(2)_MESON_EXTRA_BINARIES), \
-	        -e "/\(\[binaries\]\)/s:$$$$:\n$$(x):" \
+	        -e "/^\[binaries\]$$$$/s:$$$$:\n$$(x):" \
 	    ) \
 	    package/meson/cross-compilation.conf.in \
 	    > $$($$(PKG)_SRCDIR)/build/cross-compilation.conf