gdb: don't use C++17 namespace declaration style

gdb: don't use C++17 namespace declaration style

In a review, I suggested to use "namespace gdb::observers" instead of
two separate namespace declarations.  I didn't realize that this was a
C++17 feature, which breaks compilers that default to an earlier version
of the language, like g++ 4.8.  Change it back to two separate
declarations.

gdb/ChangeLog:

	* auto-load.h: Split namespace declaration.

Change-Id: I701537161967fbd9fcc298ff600bd072aab1251d