ext/libsamplerate: adjust config.h to be package exclusive

`config.h` being in `include` directory meant that it was included
globally. It may cause it being indluded into other package (as global
include)

Added prefixes to libsamplerate syscfg settings.
diff --git a/ext/libsamplerate/include/config.h b/ext/libsamplerate/src/config.h
similarity index 84%
rename from ext/libsamplerate/include/config.h
rename to ext/libsamplerate/src/config.h
index 756c57d..9c77c58 100644
--- a/ext/libsamplerate/include/config.h
+++ b/ext/libsamplerate/src/config.h
@@ -22,19 +22,19 @@
 
 #include "syscfg/syscfg.h"
 
-#if MYNEWT_VAL(ENABLE_SINC_BEST_CONVERTER)
+#if MYNEWT_VAL(LIBSAMPLERATE_ENABLE_SINC_BEST_CONVERTER)
 #define ENABLE_SINC_BEST_CONVERTER      1
 #endif
 
-#if MYNEWT_VAL(ENABLE_SINC_MEDIUM_CONVERTER)
+#if MYNEWT_VAL(LIBSAMPLERATE_ENABLE_SINC_MEDIUM_CONVERTER)
 #define ENABLE_SINC_MEDIUM_CONVERTER      1
 #endif
 
-#if MYNEWT_VAL(ENABLE_SINC_FAST_CONVERTER)
+#if MYNEWT_VAL(LIBSAMPLERATE_ENABLE_SINC_FAST_CONVERTER)
 #define ENABLE_SINC_FAST_CONVERTER      1
 #endif
 
-#if MYNEWT_VAL(LIBSAMPLER_NDEBUG)
+#if MYNEWT_VAL(LIBSAMPLERATE_LIBSAMPLER_NDEBUG)
 #define LIBSAMPLER_NDEBUG      1
 #endif
 
diff --git a/ext/libsamplerate/syscfg.yml b/ext/libsamplerate/syscfg.yml
index 11232a6..2858200 100644
--- a/ext/libsamplerate/syscfg.yml
+++ b/ext/libsamplerate/syscfg.yml
@@ -16,15 +16,15 @@
 # under the License.
 
 syscfg.defs:
-    ENABLE_SINC_BEST_CONVERTER:
+    LIBSAMPLERATE_ENABLE_SINC_BEST_CONVERTER:
         description: Enable SINC best converter
         value: 1
-    ENABLE_SINC_MEDIUM_CONVERTER:
+    LIBSAMPLERATE_ENABLE_SINC_MEDIUM_CONVERTER:
         description: Enable SINC medium converter
         value: 1
-    ENABLE_SINC_FAST_CONVERTER:
+    LIBSAMPLERATE_ENABLE_SINC_FAST_CONVERTER:
         description: Enable SINC fastest converter
         value: 1
-    LIBSAMPLER_NDEBUG:
+    LIBSAMPLERATE_LIBSAMPLER_NDEBUG:
         description: Define NDEBUG for resampler code (turns off asserts)
         value: 0