Avoid TfToken copy while iterating in usd/primDefinition.cpp

[nvmkuruc]

Description of Change(s)

The following warning was observed in GCC 11

warning: loop variable ‘srcField’ creates a copy from type ‘const pxrInternal_v0_23__pxrReserved__::TfToken’ [-Wrange-loop-construct]
  523 |     for (const TfToken srcField : defProp.ListMetadataFields()) {
      |                        ^~~~~~~~
note: use reference type to prevent copying
  523 |     for (const TfToken srcField : defProp.ListMetadataFields()) {
      |                        ^~~~~~~~
      |                        &

Making srcField a reference type avoids the copy of the TfToken.

Fixes Issue(s)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-8652