[librpbyteswap] byteswap_rp.h, __swabf(): Initialize the union to 0.

This doesn't affect the compiled output, but it's needed by gcc-5.4
in order to be marked as constexpr.

The uninitialized union *is* allowed as of C++20, supported in
gcc-10.1 and later.

src/librpbyteswap/byteswap_rp.h

@@ -134,7 +134,7 @@ static inline float __swabf(float f)
 	union {
 		uint32_t u32;
 		float f;
-	} u32_f;
+	} u32_f = { 0 };
 	u32_f.f = f;
 	u32_f.u32 = __swab32(u32_f.u32);
 	return u32_f.f;