exec: Remove accel/tcg/ from include paths

exec: Remove accel/tcg/ from include paths

When TCG is enabled, the accel/tcg/ include path is added to the
project global include search list. This accel/tcg/ directory
contains a header named "internal.h" which, while intented to
be internal to accel/tcg/, is accessible by all files compiled
when TCG is enabled. This might lead to problem with other
directories using the same "internal.h" header name:

  $ git ls-files | fgrep /internal.h
  accel/tcg/internal.h
  include/hw/ide/internal.h
  target/hexagon/internal.h
  target/mips/internal.h
  target/ppc/internal.h
  target/s390x/internal.h

As we don't need to expose accel/tcg/ internals to the rest of
the code base, simplify by removing it from the include search
list, and include the accel/tcg/ public headers relative to the
project root search path (which is already in the generic include
search path).

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Claudio Fontana <cfontana@suse.de>
Message-Id: <20210413081008.3409459-1-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>