fix Qt macOS build with Clang 18

    See:
    https://github.com/pnggroup/libpng/commit/893b8113f04d408cc6177c6de19c9889a48faa24.

    In a similar manner as zlib (madler/zlib#895),
    libpng contains a header configuration that's no longer valid and
    hasn't been exercised for the macOS target.

    - The target OS conditional macros are misused. Specifically
      `TARGET_OS_MAC` covers all Apple targets, including iOS, and it
      should not be checked with `#if defined` as they would always be
      defined (to either 1 or 0) on Apple platforms.
    - `#include <fp.h>` no longer works for the macOS target and results
      in a compilation failure. macOS ships all required functions in
      `math.h`, and clients should use `math.h` instead.

--- a/qtbase/src/3rdparty/libpng/pngpriv.h
+++ b/qtbase/src/3rdparty/libpng/pngpriv.h
@@ -514,18 +514,8 @@
     */
 #  include <float.h>
 
-#  if (defined(__MWERKS__) && defined(macintosh)) || defined(applec) || \
-    defined(THINK_C) || defined(__SC__) || defined(TARGET_OS_MAC)
-   /* We need to check that <math.h> hasn't already been included earlier
-    * as it seems it doesn't agree with <fp.h>, yet we should really use
-    * <fp.h> if possible.
-    */
-#    if !defined(__MATH_H__) && !defined(__MATH_H) && !defined(__cmath__)
-#      include <fp.h>
-#    endif
-#  else
-#    include <math.h>
-#  endif
+#  include <math.h>
+
 #  if defined(_AMIGA) && defined(__SASC) && defined(_M68881)
    /* Amiga SAS/C: We must include builtin FPU functions when compiling using
     * MATH=68881