samples: common: move wifi.h header to samples/common/

[mniestroj]

WiFi helper module was moved from net/wifi/ to samples/common/ in commit ee8859e3f279 ("Move net/wifi/ to samples/common/ directory"). However, header file was missed during that time. Move it now to samples/common/.

Introduce CONFIG_GOLIOTH_SAMPLES_COMMON Kconfig option, which is selected by all samples sharing common samples code. This will prevent out-of-tree applications to see such header files by default.

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