Fix ECP alternative macro names

The compile time macros enabling the initialisation and deinitialisation
in the alternative Elliptic Curve Point arithmetic implementation had
names that did not end with '_ALT' as required by check-names.sh.
diff --git a/include/mbedtls/ecp_internal.h b/include/mbedtls/ecp_internal.h
index 835b0cc..67a3de2 100644
--- a/include/mbedtls/ecp_internal.h
+++ b/include/mbedtls/ecp_internal.h
@@ -29,12 +29,13 @@
 
 unsigned char ecp_alt_grp_capable( const mbedtls_ecp_group *grp );
 
-#if defined(MBEDTLS_ECP_ALT_INIT)
+#if defined(MBEDTLS_ECP_INIT_ALT)
 int  ecp_alt_init( const mbedtls_ecp_group *grp );
 #endif
 
-#if defined(MBEDTLS_ECP_ALT_DEINIT)
+#if defined(MBEDTLS_ECP_DEINIT_ALT)
 void ecp_alt_deinit( const mbedtls_ecp_group *grp );
+#endif
 
 #if defined(MBEDTLS_ECP_RANDOMIZE_JAC_ALT)
 int ecp_randomize_jac_alt( const mbedtls_ecp_group *grp, mbedtls_ecp_point *pt,