[PJRT] Use typedef struct X {} X; idiom for PJRT_Api.

This allows forward declaration (`struct PJRT_Api;`) in C++, i.e. if you want to carry `PJRT_Api` as an opaque pointer to pass further without need to depend on the actual header.

In C, `struct X` and (typedef) `X` are in different namespaces so they can coexist.

In C++ this idiom is allowed by § 9.2.3.3 of the standard for compatibility with C.

PiperOrigin-RevId: 674324680

xla/pjrt/c/pjrt_c_api.h

@@ -2118,7 +2118,7 @@ typedef PJRT_Error* PJRT_Compile(PJRT_Compile_Args* args);
 #define _PJRT_API_STRUCT_FIELD(fn_type) fn_type* fn_type
 
 // Please modify PJRT_Api_STRUCT_SIZE if the last field of PJRT_Api is changed.
-typedef struct {
+typedef struct PJRT_Api {
   size_t struct_size;
   PJRT_Extension_Base* extension_start;