include/disas/dis-asm.h: Handle being included outside 'extern "C"'

Make dis-asm.h handle being included outside an 'extern "C"' block;
this allows us to remove the 'extern "C"' blocks that our two C++
files that include it are using.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

include/disas/dis-asm.h

@@ -9,6 +9,12 @@
 #ifndef DISAS_DIS_ASM_H
 #define DISAS_DIS_ASM_H
 
+#include "qemu/bswap.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef void *PTR;
 typedef uint64_t bfd_vma;
 typedef int64_t bfd_signed_vma;
@@ -479,8 +485,6 @@ bool cap_disas_plugin(disassemble_info *info, uint64_t pc, size_t size);
 
 /* from libbfd */
 
-#include "qemu/bswap.h"
-
 static inline bfd_vma bfd_getl64(const bfd_byte *addr)
 {
     return ldq_le_p(addr);
@@ -508,4 +512,8 @@ static inline bfd_vma bfd_getb16(const bfd_byte *addr)
 
 typedef bool bfd_boolean;
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* DISAS_DIS_ASM_H */