Change rad2deg function to static

[zayfen]

Summary

rad2deg is a very common utility function name and can easily cause duplicate symbol definition errors when linking against third-party static libraries (.a files) that happen to define a function with the same name.
rad2deg is only used within RNSVGMarkerPosition.mm itself, so there is no reason for it to have external linkage. Declaring it static restricts its visibility to the translation unit, which:

• Eliminates the risk of duplicate symbol linker errors with any external library that defines a function of the same name.
• Better communicates intent — the function is an internal implementation detail and should not be part of the public symbol table.
  Impacted area: iOS/macOS marker rendering (RNSVGMarkerPosition.mm).

Test Plan

Prerequisites

• Xcode 15+ (reproduces with the new Apple linker, which is stricter about duplicate symbols)
• A project that links react-native-svg alongside a third-party static library that also defines rad2deg

Steps to reproduce the original issue

1. Add a native static library (.a) that exports a rad2deg(double) symbol to your React Native iOS project.
2. Run xcodebuild — the build will fail with:
   duplicate symbol 'rad2deg(double)' in: libRNSVG.a (RNSVGMarkerPosition.o) .a (...) ld: 1 duplicate symbols clang: error: linker command failed with exit code 1

Verification after fix

1. Apply this patch.
2. Clean DerivedData and rebuild: yarn ios --device.
3. Build succeeds — no duplicate symbol error.

Compatibility

OS	Implemented
iOS	✅
MacOS	✅
Android	❌
Web	❌

Android and Web are not affected — the change is confined to the .mm (Objective-C++) source file used only on Apple platforms.

Checklist

• I have tested this on a device and a simulator

• I added documentation in README.md

• I updated the typed files (typescript)

• I added a test for the API in the __tests__ folder

• I have tested this on a device and a simulator

• I added documentation in README.md

• I updated the typed files (typescript)

• I added a test for the API in the __tests__ folder