freeRTOS: shrink heap by half

A 128K reservation on a device with 256K of RAM is a bit demanding.
Shrink to fix a link error in a DebugRel build.

src/FreeRTOSConfig.h

diff --git a/src/FreeRTOSConfig.h b/src/FreeRTOSConfig.h
index 63ce706..a236a19 100644
--- a/src/FreeRTOSConfig.h
+++ b/src/FreeRTOSConfig.h
@@ -71,7 +71,7 @@
 /* Memory allocation related definitions. */
 #define configSUPPORT_STATIC_ALLOCATION         0
 #define configSUPPORT_DYNAMIC_ALLOCATION        1
-#define configTOTAL_HEAP_SIZE                   (128*1024)
+#define configTOTAL_HEAP_SIZE                   (64*1024)
 #define configAPPLICATION_ALLOCATED_HEAP        0
 
 /* Hook function related definitions. */