avoid "inline" in public headers for strict c89 compatibility

while musl itself requires a c99 compiler, some applications insist on
being compiled with c89 compilers, and use of "inline" in the headers
was breaking them. much of this had been avoided already by just
skipping the inline keyword in pre-c99 compilers or modes, but this
new unified solution is cleaner and may/should result in better code
generation in the default gcc configuration.

include/sys/syscall.h

@@ -4,6 +4,10 @@
 extern "C" {
 #endif
 
+#if __STDC_VERSION__ >= 199901L || defined(__cplusplus)
+#define __inline inline
+#endif
+
 long __syscall_ret(unsigned long);
 long __syscall(long, ...);
 long syscall(long, ...);