There's a subtle difference in the internal name of siginfo_t fields
between glibc and musl. The structure itself is equivalent, so it
should suffice to add a macro to rename the field.

--- a/src/3rdparty/chromium/sandbox/linux/seccomp-bpf/trap.cc
+++ b/src/3rdparty/chromium/sandbox/linux/seccomp-bpf/trap.cc
@@ -25,6 +25,11 @@
 #include "sandbox/linux/system_headers/linux_seccomp.h"
 #include "sandbox/linux/system_headers/linux_signal.h"
 
+// musl libc defines siginfo_t __si_fields instead of _sifields
+#if !defined(__GLIBC__)
+#define _sifields __si_fields
+#endif
+
 namespace {
 
 struct arch_sigsys {