Fix narrowing error in OHOS MapKit PNG signature

The pngSignature array stored 0x89 in a plain char, which only compiles
where char is unsigned, as on arm64. On x86_64 char is signed, so 0x89
(137) exceeds CHAR_MAX and the build fails with a -Wc++11-narrowing
error.

Use a string literal instead. It yields the identical bytes, including
the trailing null terminator that the QByteArray::startsWith(const char *)
overload relies on, without any narrowing.

This amends 0e69850e51d9c8aacbf5bfc37f5f085f5e48f0cb .

Pick-to: 6.12
Task-number: QTBUG-150347
Change-Id: I54e612c3ec4157ce3425fb842f1a2c19e70f71f8
Reviewed-by: Liang Qi <liang.qi@qt.io>