Add BEGIN_TIER1_NAMESPACE around CUtlBuffer code

This is to help avoid name collisions when static linking.
(See 53bff2c)

P4:7324330

src/tier1/utlbuffer.cpp

@@ -17,6 +17,7 @@
 // memdbgon must be the last include file in a .cpp file!!!
 #include "tier0/memdbgon.h"
 
+BEGIN_TIER1_NAMESPACE
 
 //-----------------------------------------------------------------------------
 // Character conversions for C strings
@@ -1806,4 +1807,6 @@ double CUtlBuffer::GetDouble()
 	double d;
 	GET_TYPE( double, d, "%f" );
 	return d;
-}
+}
+
+END_TIER1_NAMESPACE