PalmServiceBridge: remove const from m_identifier

* there is free() call in PalmServiceBridge destructor and newer g++ doesn't like it:
  5.2.1+gitAUTOINC+7d1921a741-r0/git/Source/WebCore/platform/webos/PalmServiceBridge.cpp:150:26:
  error: invalid conversion from 'const void*' to 'void*' [-fpermissive]
  |          free(m_identifier);
  /usr/include/stdlib.h:483:13: error:   initializing argument 1 of 'void free(void*)' [-fpermissive]
  |  extern void free (void *__ptr) __THROW;

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>