gdb.base/watchpoint.{c,exp}

gdb.base/watchpoint.{c,exp}

Adjust gdb.base/watchpoint.c so that it can be built as a C++ program.

Fixes:
gdb compile failed, src/gdb/testsuite/gdb.base/watchpoint.c:33:16: error: initializer-string for array of chars is too long [-fpermissive]
   33 | char buf[30] = "testtesttesttesttesttesttestte";
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/gdb/testsuite/gdb.base/watchpoint.c:62:14: error: expected unqualified-id before 'nullptr'
   62 | struct foo5 *nullptr;
      |              ^~~~~~~

gdb/testsuite/ChangeLog:

	* gdb.base/watchpoint.c (buf): Make it 31 bytes.
	(nullptr): Rename to ...
	(null_ptr): ... this.
	* gdb.base/watchpoint.exp: Adjust to rename.