The treaty you are asking about is actually the Convention on Prohibitions or Restrictions on the Use of Certain Conventional Weapons and not the Geneva Convention.
The second part of that convention (the Protocol on Mines, Booby-Traps and Other Devices) defines a booby-trap as:
"Booby-trap" means any device or material which is designed, constructed or adapted to kill or injure, and which functions unexpectedly when a person disturbs or approaches an apparently harmless object or performs an apparently safe act.
A car-bomb is a booby-trap, and therefore a conventional weapon under the Convention.
Usage of Booby Traps
To answer another aspect of your question, the usage of booby-traps against civilians is prohibited in "non-international" conflicts:
In case of armed conflicts not of an international character occurring in the territory of one of the High Contracting Parties, each party to the conflict shall be bound to apply the prohibitions and restrictions of this Protocol.
However, this does not apply to ordinary disturbances, such as riots, but only to armed conflict (such as civil war):
This Protocol shall not apply to situations of internal disturbances and tensions, such as riots, isolated and sporadic acts of violence and other acts of a similar nature, as not being armed conflicts.
Although none of this matters in this case, because Syria is not a signatory to this Convention.