Suppress pedantic warnings for dt_btf.c

The file includes include/linux/btf.h, which has an unnamed
union.  Suppress the warning
    ISO C99 doesn?t support unnamed structs/unions [-Wpedantic]

Signed-off-by: Eugene Loh <eugene.loh@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

libdtrace/Build

@@ -93,6 +93,7 @@ libdtrace_LIBSOURCES := libdtrace-build libproc libport libcommon
 libdtrace_SECONDARY := libproc libport
 
 # Disable certain warnings for these files
+dt_btf.c_CFLAGS := -Wno-pedantic
 dt_consume.c_CFLAGS := -Wno-pedantic
 dt_debug.c_CFLAGS := -Wno-prio-ctor-dtor
 dt_cg.c_CFLAGS := -Wno-pedantic