Don't include inside the mlpack namespace.

A downstream user may try to re-include execinfo and get very confused when it silently fails.

CMake/backtrace.h.in

@@ -1,13 +1,15 @@
+/**
+ * @file backtrace.hpp
+ *
+ * Configuration file that includes backtrace headers (if they exist on this
+ * system), auto-generated by CMake.
+ */
 #ifndef __MLPACK_CORE_UTIL_BACKTRACE_HPP
 #define __MLPACK_CORE_UTIL_BACKTRACE_HPP
 
-namespace mlpack {
-
 #cmakedefine01 Backtrace_FOUND
 #if Backtrace_FOUND
   #include <@Backtrace_HEADER@>
 #endif
 
-}; //namespace mlpack
-
 #endif