gdb: add AMDGPU header files to HFILES_NO_SRCDIR
Commit 18b4d0736bc5 ("gdb: initial support for ROCm platform (AMDGPU)
debugging") missed adding these header files to the HFILES_NO_SRCDIR
list in the Makefile. Fix that now.
Change-Id: Ifd387096aef3d147b51aefa2037da5bf6373ea64