Add explicit return type to pd_func_caller macro to resolve type annotations error

[missfunmi]

Explicitly specifying the return type fixes the type annotation errors reported in #93

error[E0282]: type annotations needed
  --> src/lib.rs:84:13
   |
84 |             Ok(raw_fn())
   |             ^^^^^^^^^^^^ cannot infer type
   |
  ::: src/display.rs:25:13
   |
25 |             pd_func_caller!((*self.0).getWidth)?,
   |             ----------------------------------- in this macro invocation
   |
   = note: this error originates in the macro `pd_func_caller` (in Nightly builds, run with -Z macro-backtrace for more info)

Tested with:

• M1 Macbook Pro (2021) running macOS Sequoia 15.3.2
• rustc 1.88.0-nightly (d5b4c2e4f 2025-04-02)
• Playdate SDK 2.6.2