Add "extern" to stderr to avoid CFFI warning

UserWarning: Global variable 'stderr' in cdef():
for consistency with C it should have a storage class specifier
(usually 'extern')

Author: mgeier

sounddevice_build.py

@@ -313,8 +313,8 @@
     /* from stdio.h */
     FILE* fopen(const char* path, const char* mode);
     int fclose(FILE* fp);
-    FILE* stderr;  /* GNU C library */
-    FILE* __stderrp;  /* macOS */
+    extern FILE* stderr;  /* GNU C library */
+    extern FILE* __stderrp;  /* macOS */
 """)
 
 if __name__ == '__main__':