jni: Resolve -Wwritable-strings warnings in helium FW library variables

* fixes: ISO C++11 does not allow conversion from string literal to 'char *'

Change-Id: I76de69d5c2fef3f53c5d57a4fc11b4d8f959ed93
diff --git a/jni/android_hardware_fm.cpp b/jni/android_hardware_fm.cpp
index 79d6eae..edf9b18 100644
--- a/jni/android_hardware_fm.cpp
+++ b/jni/android_hardware_fm.cpp
@@ -83,8 +83,8 @@
 
 namespace android {
 
-char *FM_LIBRARY_NAME = "fm_helium.so";
-char *FM_LIBRARY_SYMBOL_NAME = "FM_HELIUM_LIB_INTERFACE";
+const char *FM_LIBRARY_NAME = "fm_helium.so";
+const char *FM_LIBRARY_SYMBOL_NAME = "FM_HELIUM_LIB_INTERFACE";
 void *lib_handle;
 static int slimbus_flag = 0;