hexagon: get rid of #include <generated/compile.h>

<generated/compile.h> is created (or updated) when Kbuild descends
into the init/ directory.  In parallel building from a pristine
source tree, there is no guarantee <generated/compile.h> exists when
arch/hexagon/kernel/ptrace.c is compiled.

For hexagon architecture, we know UTS_MACHINE is a fixed string
"hexagon", so let's hard-code it, like many architectures do.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Richard Kuo <rkuo@codeaurora.org>