add const spec to Event::getHeader()'s param

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@12526 d0543943-73ff-0310-b7d9-9358b9ac24b2

libs/esl/src/esl_oop.cpp

@@ -327,7 +327,7 @@ bool ESLevent::setPriority(esl_priority_t priority)
 	return false;
 }
 
-const char *ESLevent::getHeader(char *header_name)
+const char *ESLevent::getHeader(const char *header_name)
 {
 	this_check("");
 

libs/esl/src/include/esl_oop.h

@@ -57,7 +57,7 @@ class ESLevent {
 	virtual ~ESLevent();
 	const char *serialize(const char *format = NULL);
 	bool setPriority(esl_priority_t priority = ESL_PRIORITY_NORMAL);
-	const char *getHeader(char *header_name);
+	const char *getHeader(const char *header_name);
 	char *getBody(void);
 	const char *getType(void);
 	bool addBody(const char *value);