gcc: fix implication order in assertion

In 6812dd98c4 I mistakenly had the
implication order reversed.  This commit corrects that mistake.

The original assertion (which is correct) was the following, which
asserts that if you enable the GDB plugin, you must enable plugins
generally (there is shared infrastructure):

```
assert enableGdbPlugin -> enablePlugin;
```

When the option name was changed to `disableGdbPlugin`, I
incorrectly wrote:

```
assert disableGdbPlugin -> enablePlugin;
```

And then again incorrectly wrote:

```
assert disableGdbPlugin -> !enablePlugin;
```

This commit uses the correct equivalent for the first statement,
which is the contrapositive:

```
assert !enablePlugin -> disableGdbPlugin;
```

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -27,7 +27,7 @@
 , disableBootstrap ? stdenv.targetPlatform != stdenv.hostPlatform
 }:
 
-assert disableGdbPlugin -> !enablePlugin;
+assert !enablePlugin -> disableGdbPlugin;
 assert langJava -> lib.versionOlder version "7";
 
 # Note [Windows Exception Handling]