Mark folly_async_stack_root_tls_key inline

Summary:
I'm working on adding async stack trace support to Unifex and we'd like the
Unifex and Folly implementations to work together without either library
depending on the other. To that end, this diff marks
`folly_async_stack_root_tls_key` `inline` so that, when Unifex declares the same
global variable, also `inline`, the linker de-duplicates them rather than
failing with a duplicate symbol error.

Reviewed By: jesswong

Differential Revision: D57456364

fbshipit-source-id: f8a3b3dbde8fd3b912e4db37c19fccff9a695785

third-party/folly/src/folly/tracing/AsyncStack.cpp

@@ -43,7 +43,7 @@
 extern "C" {
 // Current pthread implementation has valid keys in range 0 .. 1023.
 // Initialise to some value that will be interpreted as an invalid key.
-pthread_key_t folly_async_stack_root_tls_key = 0xFFFF'FFFFu;
+inline pthread_key_t folly_async_stack_root_tls_key = 0xFFFF'FFFFu;
 }
 
 #endif // FOLLY_ASYNC_STACK_ROOT_USE_PTHREAD