Revert "Use inline keyword without underscores consistently"

This reverts commit 58f06e11122723d68c5cb393aeb4ed1c7a2e7bfe.

Bare `inline` is not ANSI C compatible, and NDK users may be using
that.

Test: treehugger
Change-Id: I82c5424522142001cd59da63ef3fd440014451ad
diff --git a/libc/include/bits/threads_inlines.h b/libc/include/bits/threads_inlines.h
index 074e1ca..5878e0a 100644
--- a/libc/include/bits/threads_inlines.h
+++ b/libc/include/bits/threads_inlines.h
@@ -38,7 +38,7 @@
 
 __BEGIN_DECLS
 
-static inline int __bionic_thrd_error(int __pthread_code) {
+static __inline int __bionic_thrd_error(int __pthread_code) {
   switch (__pthread_code) {
     case 0: return 0;
     case ENOMEM: return thrd_nomem;