gdb: Fix phony iconv build

gdb: Fix phony iconv build

Cross building gdb for mingw32 on Fedora 23 fails with:

  x86_64-w64-mingw32-g++ -g -O2  [...]  ../../src/gdb/charset.c
  In file included from ../../src/gdb/charset.c:21:0:
  ../../src/gdb/charset.h:134:3: error: 'iconv_t' does not name a type
     iconv_t m_desc;
     ^
  ../../src/gdb/charset.c: In constructor 'wchar_iterator::wchar_iterator(const gdb_byte*, size_t, const char*, size_t)':
  ../../src/gdb/charset.c:600:3: error: 'm_desc' was not declared in this scope
     m_desc = iconv_open (INTERMEDIATE_ENCODING, charset);
     ^
  ../../src/gdb/charset.c: In destructor 'wchar_iterator::~wchar_iterator()':
  ../../src/gdb/charset.c:607:7: error: 'm_desc' was not declared in this scope
     if (m_desc != (iconv_t) -1)
	 ^
  ../../src/gdb/charset.c: In member function 'int wchar_iterator::iterate(wchar_iterate_result*, gdb_wchar_t**, const gdb_byte**, size_t*)':
  ../../src/gdb/charset.c:633:25: error: 'm_desc' was not declared in this scope
	 size_t r = iconv (m_desc, &inptr, &m_bytes, &outptr, &out_avail);
			   ^

This is a regression caused by commit cda6c55bd399 (Turn wchar
iterator into a class).  The problem is that iconv_t is now exposed in
gdb/charset.h, while before it was only used in gdb/charset.c.

gdb/charset.c, under #ifdef PHONY_ICONV, does:

 #undef iconv_t
 #define iconv_t int

So it seems the simplest is to use 'int' in the header file too.

gdb/ChangeLog:
2016-10-17  Pedro Alves  <palves@redhat.com>

	* charset.h (class wchar_iterator) [PHONY_ICONV] <m_desc>: Use
	'int' as type.