constexpr in camera CompParameters

Summary:
Behavior-preserving optimization (constexpr) for `geometry/camera.h`:
- Mark `CompParameters::operator()` `constexpr` (pure comparator, only `static_cast` + `<`, literal `size_t` return). Additive; the operator was already implicitly inline.

No observable behavior change. Verified by build + tests (205 passed).

___

Differential Revision: D108284669

fbshipit-source-id: cf43011ea0cb483306efe40c904d589edefa6bc1

Author: Fabian Schenk

opensfm/src/geometry/camera.h

@@ -28,7 +28,8 @@ class Camera {
   };
 
   struct CompParameters {
-    size_t operator()(const Parameters& p1, const Parameters& p2) const {
+    constexpr size_t operator()(const Parameters& p1,
+                                const Parameters& p2) const {
       return static_cast<size_t>(p1) < static_cast<size_t>(p2);
     }
   };