Fix ODR violation for IsFloatType_v

With recent GCC 15 trunk, I started to see:
```
ld: .../kwalletentry.cc.o:(.rodata+0x0): multiple definition of `QtPrivate::IsFloatType_v<_Float16>';
	src/runtime/kwalletd/backend/CMakeFiles/KF6WalletBackend.dir/cbc.cc.o:(.rodata+0x0): first defined here
```

The issue is that constexpr is only implicitly inline for functions or
static data members [0], so the constexpr IsFloatType_v specialization
here causes an ODR violation.

Explicitly mark the specialization as inline constexpr.

[0] http://eel.is/c++draft/dcl.constexpr#1.sentence-3

Fixes: 4b755bc
Pick-to: 6.8 6.7
Change-Id: Ie9257138f6d1218ca0a91f5d114aab2483cb275b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qcomparehelpers.h

@@ -774,7 +774,7 @@ constexpr bool IsFloatType_v = std::is_floating_point_v<T>;
 
 #if QFLOAT16_IS_NATIVE
 template <>
-constexpr bool IsFloatType_v<QtPrivate::NativeFloat16Type> = true;
+inline constexpr bool IsFloatType_v<QtPrivate::NativeFloat16Type> = true;
 #endif
 
 } // namespace QtPrivate