dub.sdl: Use compiler specific -preview=in. Disable it for GDC.

Having `-preview=in` in dflags without a platform specification
leads to errors with gdc:
```
gdc: error: unrecognized command-line option ‘-preview=in’; did you mean ‘-fpreview=in’?
```

However, using the proper `-fpreview=in` for gdc causes an ICE:
```
/usr/lib/gcc/x86_64-pc-linux-gnu/13/include/d/std/math/exponential.d:587:30: internal compiler error: in layout_aggregate_type, at d/types.cc:562
  587 |             enum maxOdd = pow(2.0L, real.mant_dig) - 1.0L;
```

Leave `-preview=in` enabled for dmd and ldc and wait for a fix
before enabling it for gdc.

Signed-off-by: Andrei Horodniceanu <[email protected]>

dub.sdl

@@ -7,7 +7,10 @@ license "MIT"
 
 targetPath "bin"
 
-dflags "-preview=in"
+dflags "-preview=in" platform="dmd"
+dflags "-preview=in" platform="ldc"
+//Disabled due to ICEs in gdc.
+//dflags "-fpreview=in" platform="gdc"
 // Deprecated module(s)
 excludedSourceFiles "source/dub/packagesupplier.d"