libs/libc/audio/libsrc: update header location

update Make.defs with the correct path to the library header and
make sure it can be found

libs/libc/audio/libsrc/Make.defs

@@ -27,7 +27,7 @@ libsamplerate:
 	$(Q) curl -L https://codeload.github.com/libsndfile/libsamplerate/zip/master -o libsamplerate.zip
 	$(Q) unzip -o libsamplerate.zip
 	$(Q) mv libsamplerate-master libsamplerate
-	$(Q) cp -rf libsamplerate/src/samplerate.h $(TOPDIR)$(DELIM)include$(DELIM)nuttx$(DELIM)audio$(DELIM)
+	$(Q) cp -rf libsamplerate/include/samplerate.h $(TOPDIR)$(DELIM)include$(DELIM)nuttx$(DELIM)audio$(DELIM)
 
 context:: libsamplerate
 
@@ -38,6 +38,8 @@ CSRCS += src_zoh.c
 
 CFLAGS += -DPACKAGE=\"$(PACKAGE)\" -DVERSION=\"$(VERSION)\"
 
+CFLAGS += ${shell $(INCDIR) "$(CC)" $(TOPDIR)$(DELIM)include$(DELIM)nuttx$(DELIM)audio}
+
 ifeq ($(CONFIG_SINC_FAST_CONVERTER),y)
 CFLAGS += -DENABLE_SINC_FAST_CONVERTER
 endif