Include aarch32-tdep.h in aarch32-tdep.c

Include aarch32-tdep.h in aarch32-tdep.c

The error below shows that aarch32-tdep.c is missing an include for
aarch32-tdep.h, add it.

      CXX    aarch32-tdep.o
    /home/smarchi/src/binutils-gdb/gdb/aarch32-tdep.c: In function ‘const target_desc* aarch32_read_description()’:
    /home/smarchi/src/binutils-gdb/gdb/aarch32-tdep.c:27:1: error: no previous declaration for ‘const target_desc* aarch32_read_description()’ [-Werror=missing-declarations]
     aarch32_read_description ()
     ^~~~~~~~~~~~~~~~~~~~~~~~

Putting the include of aarch32-tdep.h early in aarch32-tdep.c gives us
an error about target_desc not being defined.  Indeed, aarch32-tdep.h
uses target_desc without forward-declaring it or including the proper
header.  Add a forward-declaration for it.

gdb/ChangeLog:

	* aarch32-tdep.c: Include aarch32-tdep.h.
	* aarch32-tdep.h: Forward-declare struct target_desc.

Change-Id: Ica4be4de0fbd7f22d56a29a40fbf0a31b5abdb16