Export the external-semaphore stubs: include the internal libhsakmt.h.

src/extsem.c includes only the PUBLIC hsakmt/hsakmt.h, where HSAKMTAPI expands
to nothing on Linux. Every other source file includes the INTERNAL src/
libhsakmt.h, which redefines it:

    #define HSAKMTAPI __attribute__((visibility ("default")))

The library is compiled with -fvisibility=hidden, so extsem.c's four functions
end up STV_HIDDEN and cannot be exported -- even though they ARE listed in
src/libhsakmt.ver's global: section. A version script cannot promote a hidden
symbol, so the entries silently have no effect and consumers fail to link with

    undefined reference to `hsaKmtImportExternalSemaphore'
    undefined reference to `hsaKmtDestroyExternalSemaphore'
    undefined reference to `hsaKmtQueueSignalExternalSemaphore'
    undefined reference to `hsaKmtQueueWaitExternalSemaphore'

Verified 2026-08-29: extsem.c.o has bind=GLOBAL vis=HIDDEN for all four, while
e.g. openclose.c's hsaKmtOpenKFD has vis=DEFAULT.
--- a/src/extsem.c
+++ b/src/extsem.c
@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: MIT
  */
 
+#include "libhsakmt.h"
 #include "hsakmt/hsakmt.h"
 
 HSAKMT_STATUS HSAKMTAPI hsaKmtImportExternalSemaphore(