spi_flash: extern spi_flash_chip_generic_timeout

spi_flash_generic_timeout is currently defined in the header file,
which makes it a common symbol between translation units where this
header file is included. It is also defined in
spi_flash_chip_generic.c.

Add an explicit `extern` to prevent multiple definition errors.

components/spi_flash/include/spi_flash_chip_generic.h

@@ -408,4 +408,4 @@ esp_err_t spi_flash_chip_generic_suspend_cmd_conf(esp_flash_t *chip);
 esp_err_t spi_flash_chip_generic_read_unique_id_none(esp_flash_t *chip, uint64_t* flash_unique_id);
 
 /// Default timeout configuration used by most chips
-const flash_chip_op_timeout_t spi_flash_chip_generic_timeout;
+extern const flash_chip_op_timeout_t spi_flash_chip_generic_timeout;