[gdb/testsuite] Fix missing uint8_t in gdb.fortran/logical.exp

[gdb/testsuite] Fix missing uint8_t in gdb.fortran/logical.exp

With test-case gdb.fortran/logical.exp, I run into:
...
(gdb) PASS: gdb.fortran/logical.exp: var=l: get hexadecimal valueof "&l"
set *((uint8_t *) 0x7fffffffd2bc) = 0xff^M
No symbol "uint8_t" in current context.^M
(gdb) FAIL: gdb.fortran/logical.exp: var=l: byte 0: set contents of byte at offset 0
...

Fix this by using the fortran-native type character instead.

Tested on x86_64-linux, with gcc 7.5.0.

gdb/testsuite/ChangeLog:

2020-03-11  Tom de Vries  <tdevries@suse.de>

	* gdb.fortran/logical.f90: Define variable with character type.
	* gdb.fortran/logical.exp: Use character type instead of uint8_t.