Fix c++ build

src/precomputed_ecmult.h

@@ -7,6 +7,10 @@
 #ifndef SECP256K1_PRECOMPUTED_ECMULT_H
 #define SECP256K1_PRECOMPUTED_ECMULT_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include "group.h"
 #if defined(EXHAUSTIVE_TEST_ORDER)
 #if EXHAUSTIVE_TEST_ORDER == 13
@@ -24,4 +28,8 @@ extern const secp256k1_ge_storage secp256k1_pre_g[ECMULT_TABLE_SIZE(WINDOW_G)];
 extern const secp256k1_ge_storage secp256k1_pre_g_128[ECMULT_TABLE_SIZE(WINDOW_G)];
 #endif /* defined(EXHAUSTIVE_TEST_ORDER) */
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* SECP256K1_PRECOMPUTED_ECMULT_H */