Move `extern "C" specifications for C++ to where they are needed

This should fix errors when compiling C++ code with modules enabled on clang.

include/jemalloc/jemalloc.sh

@@ -6,9 +6,6 @@ cat <<EOF
 #ifndef JEMALLOC_H_
 #define JEMALLOC_H_
 #pragma GCC system_header
-#ifdef __cplusplus
-extern "C" {
-#endif
 
 EOF
 
@@ -21,8 +18,5 @@ for hdr in jemalloc_defs.h jemalloc_rename.h jemalloc_macros.h \
 done
 
 cat <<EOF
-#ifdef __cplusplus
-}
-#endif
 #endif /* JEMALLOC_H_ */
 EOF