[gdb/testsuite] Require gnatmake-8 for gdb.ada/mi_prot.exp

With gcc-7, I run into:
...
gcc -c -I./ -gnata -Isrc/gdb/testsuite/gdb.ada/mi_prot -g -lm -I- \
  src/gdb/testsuite/gdb.ada/mi_prot/prot.adb^M
prot.adb:21:04: info: "Obj_Type" is frozen here, aspects evaluated at this \
  point^M
prot.adb:23:09: visibility of aspect for "Obj_Type" changes after freeze \
  point^M
gnatmake: "src/gdb/testsuite/gdb.ada/mi_prot/prot.adb" compilation error^M
compiler exited with status 1
  ...
FAIL: gdb.ada/mi_prot.exp: compilation prot.adb
...

Fix this by requiring gnatmake-8 for this test-case.

Tested on x86_64-linux, with gnatmake-7, gnatmake-8 and gnatmake-11.

gdb/testsuite/ChangeLog:

2020-07-24  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26293
	* gdb.ada/mi_prot.exp: Require gnatmake-8.