math: fir: move a file to where it's needed

fir_common.c is only needed for modular LLEXT builds, move it to the
fir_llext/ directory.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Author: lyakh

src/math/fir_llext/CMakeLists.txt

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 sof_llext_build("fir"
-	SOURCES ../fir_common.c
+	SOURCES fir_common.c
 		../fir_generic.c
 		../fir_hifi2ep.c
 		../fir_hifi3.c