AK: Remove signbit definition to prevent conflict in FixedPoint

Author: https://github.com/ghost
Commit: https://github.com/SerenityOS/serenity/commit/e35a858bde0
Pull-request: https://github.com/SerenityOS/serenity/pull/17623
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/elcuco
Reviewed-by: https://github.com/kleinesfilmroellchen ✅
Reviewed-by: https://github.com/linusg

AK/FixedPoint.h

@@ -16,6 +16,11 @@
 #    include <AK/Math.h>
 #endif
 
+// Solaris' definition of signbit in math_c99.h conflicts with our implementation.
+#ifdef AK_OS_SOLARIS
+#    undef signbit
+#endif
+
 namespace AK {
 
 // FIXME: this always uses round to nearest break-tie to even